Angora_Creek_Final_Design_Report_2-9-2012.pdf

Prepared For:

U. S. Forest Service, Lake Tahoe Basin Management Unit

Prepared by:

River Run Consulting, in association with:

Waterways Consulting

Northwest Hydraulic Consultants

Joan Reynolds Botany

Angora Creek Stream Restoration Design Report

Angora Creek Stream Restoration Design Report

Prepared For:

U. S. Forest Service Lake Tahoe Basin Management Unit South Lake Tahoe, CA

February 9, 2012

Prepared By:

River Run Consulting P. O. Box 362 Cedarville, CA 96104

Waterways Consulting, Inc.

403B Swift Street Santa Cruz, CA 95060

Northwest Hydraulic Consultants, Inc.

419 Mason St, Suite 200A Vacaville, CA 95688

ANGORA CREEK STREAM RESTORATION PROJECT DESIGN REPORT, FEBRUARY 9, 2012

Table of Contents 1 Introduction

1.1 Overview of Study

2 Watershed and Stream Setting

2.1 Geology

2.2 Landform Development

2.3 Soils

2.3.1 Site Soil Investigations: Methods

2.3.2 Results

2.4 Surface Water Hydrology

2.4.1 Data Availability

2.4.2 Flood Frequency Analysis

2.4.3 Flow Duration

2.4.4 Low and Base Flows

2.5 Ground Water Hydrology

2.5.1 Groundwater Inputs to the Valley Bottom

2.5.2 Meadow Soils and Groundwater

2.5.3 Groundwater and the Stream

2.6 Floodplain Topography

2.7 Vegetation Community

2.8 Watershed Sediment Supply and Transport

2.9 Channel Characteristics

2.9.1 Functional Reaches

2.9.2 Reach 1

2.9.3 Reaches 2 and 3

2.9.4 Entrenchment

2.9.5 Woody Debris

2.9.6 Substrate

2.9.7 Streambank Stability

2.10 Fluvial Geomorphic Function

2.10.1 Expected Channel Form

2.10.2 Channel-Forming Processes

2.11 Forest Encroachment

2.11.1 Natural or Human-Caused?

ANGORA CREEK STREAM RESTORATION PROJECT FINAL DESIGN REPORT, FEBRUARY 9, 2012

2.12 Aquatic and Riparian Habitat

3 Human Disturbance

3.1 Effects of Human Disturbance

3.1.1 Forest Management

3.1.2 Agriculture

3.1.3 Modern Road Construction

4 Description of Recommended Restoration Actions

4.1 Potential Restoration Approaches

4.1.1 Streambank Stabilization

4.1.2 Wood Incorporation Into Existing Channel

4.1.3 Channel Reconstruction

4.2 Reach 3 Design

4.2.1 Design Approach

4.2.2 Planform

4.2.3 Channel Capacity

4.2.4 Incorporating Woody Debris

4.2.5 Grade Control

4.2.6 Streambank Stabilization

4.2.7 Substrate

4.2.8 Fill of Existing Channel

4.2.9 Construction Considerations

4.3 Reach 2 Design

4.4 Monitoring Recommendations

4.4.1 Hydrology

4.4.2 Constructed Project Stability

4.4.3 Aquatic Habitat

4.4.4 Fish

4.4.5 Aquatic Macroinvertebrates

5 References i

List of Figures

1.1 Project location

2.1 Surface geology of the Angora Creek

watershed

2.2 Landform development

2.3 Location of soil surveys

2.4 Soils overview

2.5 Hydrology: basin hypsography and

hydrograph

2.6 Soil saturation in soil pits

2.7 Influence of soils on groundwater

2.8 Meadow topography

2.9 Watershed sediment survey locations

2.10 Channel reaches

2.11 Hydraulic function, Reaches 2 and 3

2.12 Stream channel incision model

2.13 Map of woody debris occurring the channel

2.14 Map of stream substrate condition

2.15 Historic changes in forest cover

2.16 Sacramento precipitation

2.17 Habitat quality of the existing channel

3.1 Historic aerial photos of t he project area

ii

List of Tables

2.1 Typical profile, Tahoe Silt Loam

2.2 Spot observations of flow

2.3 Annual series of spot flow observation annual

maxima

2.4 Estimates of discharge for frequently occurring

floods

2.5 Two estimates of the number of days flows

exceed floods of various recurrence intervals

2.6 Mean monthly discharge, USFS spot

observations

2.7 Watershed sediment survey notes

2.8 Stream reach characteristics

2.9 Geomorphic characteristics of Reaches 2 and

2.10 Results of the analysis of the effects of woody

debris

1 Introduction This report describes the design basis for a project to restore ecosystem function in a segment of Angora Creek located on U. S. Forest Service land near South Lake Tahoe, CA (Figure 1.1).

Several human land uses, beginning around the end of the 19th century, have impacted this portion of the creek, including logging and associated roads, livestock grazing and irrigation, and subdivision development. These actions had adverse effects on aquatic and riparian habitat. The main objective of this restoration project is to reverse the negative ecosystem impacts of past human land use.

Angora Creek drains a 5.8 square mile watershed on the south shore of Lake Tahoe, entering the Upper Truckee River on the Lake Tahoe Golf Course (Figure 1.1). The project area encompasses about 27 acres of meadow and contains 2,500 lineal ft of stream. Watershed area at the downstream end of the project area is 2.25 square miles. Elevation at the lower end of the project area is 6,350 ft; the top of the watershed is Echo Peak at around 8,895 ft.

In 2007, a large forest fire burned much of the upper portion of the watershed, up to the western boundary of the project area. Most of the understory and many trees in the meadow in the project area burned. The fire has had a significant impact on many aspects of the ecosystem, especially hydrologic function.

1.1 Overview of Study

The primary objective of this study was to develop restoration plans for the project area, based on an evaluation of the project area and surrounding watershed. The following surveys were performed as part of this study:

• Topographic survey. A topographic survey of the channel and surrounding meadow, conducted with a total station.

• Watershed Reconnaissance Survey. An ocular survey of potential sources of sediment in the watershed, sediment supply to the channel, and rates of sediment transport in the channel.

• Soil Survey. A survey of soil characteristics at 48 locations within the project area meadow, using a soil auger.

• Substrate Survey. Ocular estimates of substrate composition within the channel at numerous locations with the project area.

• Woody Debris Surveys. Included both a survey of all major woody debris accumulations in the project and detailed topographic surveys of wood in two detailed study reaches of the stream.

Extensive hydrologic, hydraulic, and various engineering and geomorphic analyses were also performed in support of restoration design.

2 Watershed and Stream Setting

2.1 Geology

Rocks underlying the Angora Creek watershed are primarily granitic (Figure 2.1), with the exception of small outcrops of altered sedimentary and volcanic rocks near the watershed boundary. Due to the weathering characteristics of local granites, sand is abundant and an important component of the stream’s sediment load.

Due to extensive glaciation, surface geological units throughout most of the watershed are sediment deposits resulting from glacial erosion and sediment transport processes. Tahoe till moraines comprise the upper half of the project area and most of the upper watershed. Pre- Tahoe moraines underlie the lower half of the project area, and older lake deposits and outwash deposits formed during glacial periods are found downstream of the project area.

2.2 Landform Development

The geologic map provides a large-scale picture of surface geology. On smaller spatial scales, however, the uniform morainal deposits shown underlying the project area have been extensively modified by localized glacial processes such as outwash fluviation and short-and long-term lacustrine development, as well as more modern fluvial reworking of older glacial deposits. To more thoroughly understand how these processes have influenced the structure of landforms in the project area, a review of the probable Holocene development of the Angora watershed is illustrative.

Figure 2.2 is a shaded topographic model of the Upper Truckee River watershed. Based on moraine deposits, the large, blue-shaded arrows represent the likely extent and movement of glacial ice during the Tahoe glaciation. The glacier occupying the current valley of the Upper Truckee River was the largest glacier in the south shore region due to extensive watershed area, but a prominent cirque indicates that a substantial glacier also formed in the Angora Creek watershed. At glacial maxima, these two glaciers likely joined near the project area (outlined in red in Figure 2.2).

The massive Upper Truckee glacier formed large lateral moraines, clearly seen in the topographic model on both the east and west sides of the current river downstream of Meyers.

On the west side of the current river, the large moraine has strongly influenced tributary drainage from Echo Creek north to Angora Creek. All major tributaries in this area are deflected to the north or south; the yellow lines in Figure 2.2 represent the straighter drainage paths that would be expected in the absence of the moraine. Clearly, deposition of the moraine blocked drainage paths and forced the tributaries to develop new drainages through the moraine.

As the glaciers receded, development of the Angora Creek drainage in the moraine was likely characterized by large-scale erosion and deposition. Angora Creek was probably blocked by ice at some times, or moraines at others. Lakes or ponds which formed from the blockages would have been areas of fine sediment deposition in still waters, and coarser deposits in deltaic areas. The creek also had to develop a valley through the new moraine, and must have extensively eroded morainal deposits. Debris flow and other forms of hillslope mass movements would have been common near steep valley walls, moraines, or where channel erosion was significant, providing sediment for deposition on the valley floor.

Underlying the project area is therefore a complex assemblage of sedimentary deposits, ranging from subaerial glacial tills to subaqueous lacustrine deposits to deltaic and fluvial deposits.

Given the chaotic nature of drainage development through the moraine, large sedimentary deposits created during single events (debris flow, landslide, etc.) or under conditions that do not occur today (lacustrine or glacial settings) probably underlie much of the project area.

2.3 Soils

Soil surveys by the Natural Resources Conservation Service (NRCS) classify soils within the meadow as Tahoe Complex (Web Soil Survey), map unit #7041, with Tahoe Silt Loam and Tahoe Silt Loam, wet variant comprising 80 percent of the complex. Table 2.1 gives the description of the typical profile for Tahoe Silt Loam.

In the NRCS survey, this soil is described as being very poorly drained, with a high range in hydraulic conductivity and a depth to water table of 0-12 inches. The Tahoe Silt Loam wet variant is described as having a depth to the water table of 0- 10 inches, and having frequent flooding and ponding.

We did further site investigations of soils to better understand the geomorphic processes responsible for formation and maintenance of the meadow, and to evaluate potential opportunities and constraints for various restoration alternatives.

Table 2.1. Typical Profile, Tahoe Silt Loam.

Depth (inches) Texture

0-3 Moderately decomposed plant material

3-15 Mucky silt loam 15-20 Gravelly coarse sand 20-30 Mucky silt loam 30-49 Loam 49-59 Loamy sand

2.3.1 Site Soil Investigations: Methods

Forty-six soil borings were made with a six-foot hand auger throughout the meadow in July, 2010.

In addition, soil profiles in exposed streambanks were examined at eleven locations throughout the project reach. Figure 2.3 shows the locations of the borings and streambank profiles. The borings were organized to assess conditions throughout the project reach and to gain information about variations within cross-valley transects (Borings 3-8, 9-13, and 14-17).

Individual horizons within each boring or streambank exposure were delineated based on consistency in color and texture. Depth at beginning and end of the horizons was recorded, as well as texture and color. Depth to saturation was recorded at each site. The majority of borings were between four and five feet in depth; saturated soils and the presence of gravel limited the depth of sampling in most cases.

When comparing our soil notes to USDA published data, it is important to note that we used the term “peat-rich,” or “peaty” to indicate organic-rich horizons. While there were some minimally decomposed plant materials in these horizons (the plant parts could still be recognized), most of the organic material was moderately to highly decomposed. Based on these characteristics, http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx the USDA (Soil Survey Division Staff, 1993) uses the term mucky instead. Peaty is reserved for soils where the plant materials are only minimally decomposed.

Our method of describing the texture of these organic horizons also differed slightly from that of the USDA. Horizons with sufficient organic matter to render them nearly black had a silty feel to them, imparted by the organics. However, because we were interested in the relative distribution of mineral particles of different textures, we attempted to identify the texture of just the mineral fraction of the horizon and we used this texture to classify the horizon. For clarity, we added the adjective peaty. As a result, a horizon we identified as a peaty loam would be the equivalent of mucky silt loam using the USDA system. We continue to refer to mucks as peat because some peat does indeed occur and also because it more easily conveys a definition to the reader.

2.3.2 Results

On-site soil investigations showed that the soils were generally consistent with the typical profile with respect to textures, with one major exception. Gravel was found almost solely in the upper portion of the project and was absent elsewhere, except for the bottom of one pit near the lower end of the project area, and at the edge of the meadow on the toe of hillslopes (Figure 2.4). Sandy textures, a major component of the NRCS typical profile, were common throughout the meadow, as were peats or mucks.

While textures from our surveys were generally consistent with the NRCS profile, the depth and thickness of various horizons were not at all consistent with the profile or among sampling sites.

Soils are highly heterogeneous within the valley bottom, both horizontally and vertically. The typical profile consisted of relatively thin horizons of loams to sandy loams banded with layers of organic rich, peat-type deposits which were black and had a silty texture. Both the peat and sandy horizons could be very narrow, less than 0.1 feet thick in many instances, and only occasionally were they thicker than 0.8 feet. At one of the streambank profiles, there were seven distinct horizons. We found little lateral continuity of individual horizons. Examination of the streambanks often showed individual horizons of sandy loams or peat enlarging or thinning laterally.

The exception to the above theme is the presence of a common soil horizon we described as sandy loam with clay. This material was well-distributed throughout the project area (Figure 2.4).

It tended to be in the lower portion of the soil profile, with an average depth to the top of the unit of 2.1 feet. It was typically at least one foot thick.

The sandy loam with clay horizon has coarse sand or small gravel scattered through a matrix of finer particles. This horizon is massive in terms of its structure, and there is no evident banding suggestive of routine fluvial deposition. Small bits of charcoal are common throughout. The clay content, although relatively low, is high enough that it resists erosion where it is exposed in the stream channel. Contacts with soil horizons above this unit were typically quite abrupt and appeared to be erosion surfaces. These factors suggest that the unit, which underlies much of the project area, is a depositional feature, perhaps lacustrine. The presence of clay within this layer suggests that some weathering has occurred in-situ. The high water table, which restricts downward water movement, has probably limited the translocation of clays formed by weathering within this layer and in overlying materials.

We found no evidence of Mazama ash, or other ash layers, either in the borings or in streambank soil exposures. This may be due to the depth of our samples, which did not exceed six feet, and were typically five feet or less from meadow surface. Charcoal was common in all pits and at all depths. In one streambank profile, four horizons contained charcoal within the top 2.3 feet.

Overall, the soil borings reflect processes of localized peat (muck) formation and small-scale deposition of sands. There is a trend for more and thicker peat-rich horizons in the lower portions of the meadow and coarse and thicker sand deposits in the upper portion. Fire appears to have been a common occurrence, based on the abundance of charcoal. In some locations we found occurrences of very silty material, orange-brown in color, which may have been ash slurry in areas formerly occupied by large roots.

2.4 Surface Water Hydrology

Angora Creek is a snowmelt dominated stream, typical of streams within the Lake Tahoe Basin.

Streamflow is generated principally by snowmelt runoff, although the stream also responds to fall and spring rainfall and the occasional summer thunderstorm. The watershed extends from an elevation of approximately 6,350 feet at Lake Tahoe Boulevard to 8,895 feet at Echo Peak.

The nearest climate station with a long period of record is the Meyers Fire Station operated by El Dorado County. The station is located approximately 2.6 miles southeast of the project site at a similar elevation. The annual mean precipitation there, based on 32 years of record, is 31.6 inches (GMA 2003). Precipitation increases rapidly with elevation. The highest rain gauge in the vicinity is the USDA NRCS Echo Peak SNOTEL site, which is at an elevation of 7,670 feet. The gauge is located within the Angora Lakes watershed. Based on 32 years of record, the annual mean precipitation is 57.5 inches (http://www.wcc.nrcs.usda.gov/snow), nearly double the precipitation received on the project site.

The topography and orientation of the watershed affects numerous hydrologic factors, such as the rate of snowmelt, evaporation rates, and vegetation distribution. Topographically, the watershed is somewhat unique because of its asymmetry. The basin long axis trends generally from southwest to northeast (see Figure 1.1 for a topographic map). A high ridgeline, Angora Ridge, runs along most of the northwestern margin of the basin, with elevations from 7,250 to 8,585 feet at Angora Peak. In contrast, the southeasterly margin is from 1,000 to 2,000 feet lower throughout most of the watershed. Another feature of topography important to hydrologic function is that the South Fork of the creek, which drains Seneca pond and areas above the pond, has a much lower watershed than the main stem of the stream, which originates at Angora Lakes. Figure 2.5 shows a hypsometric curve of the cumulative basin area (in percent) as a function of elevation.

The mean basin elevation is relatively low at approximately 6,800 feet, showing the influence of the tributary’s lower watershed and the fact that the lower half of the watershed is in an elevation band of only 400 feet, whereas the remaining half spans approximately 1,100 feet. The predominant aspect is east-facing. The lower elevation area gently slopes to the north. The mid elevation area below Angora ridge has a southeast aspect, and the highest elevations have northern to eastern aspects. The combination of the relatively low mean elevation and watershed orientation likely results in a dampened snowmelt hydrograph, but one that probably has substantial snowmelt runoff relatively early in the season. The highest elevations, which http://www.wcc.nrcs.usda.gov/snow contain the bulk of the snow load on a per acre basis, have east to north aspects, which extends the snowmelt hydrograph into summer during normal years.

For this project, our primary emphasis in hydrologic analysis is to support restoration design. Our analysis had two primary objectives: estimate the magnitude of peak flows with various recurrence intervals, and; estimate the duration and timing of occurrence of specific discharges.

2.4.1 Data Availability

Streamflow in Angora Creek has not been continuously monitored by the USGS, but several local efforts have gathered intermittent data. Currently, El Dorado County operates a continuous flow gage a short distance above Lake Tahoe Boulevard, originally established in 2002. Graham Matthews & Associates (GMA) installed the gage and apparently El Dorado County took over operations in 2003. Data for the period April-September 2002, collected by GMA, were published as part of a report on a stream restoration project and were available for this analysis.

Data collected since then by the County has not been published and was not available at the time of this report.

The Forest Service has taken individual streamflow observations, as opposed to continuously recording measurement of stage, at a location near the lower end of the project area. These “snapshot” individual measurements were taken beginning in 1991 and extending to the present (June, 2011). Data were provided by the Lake Tahoe Basin Management Unit (Sarah Howell, via e-mail, June, 2011). Observations were collected throughout the period 1991-2000, with a variable number of observations each year. There is then a large gap, and another set of observations in 2007. Another gap occurs till the present year; a current, more comprehensive, set of measurements are being made during the 2011 snowmelt runoff season. In those years where data was routinely collected, measurements were made on approximately a 7-day interval.

2.4.2 Flood Frequency Analysis

2.4.2.1 Methods

The frequency of occurrence of a given peak flow is typically estimated in a couple of ways where continuous flow records are not available. First, regional regression equations have been developed to estimate peak flow frequency based on drainage basin characteristics. Second, flood frequency can be extrapolated from nearby, similar basins with continuous flow records by adjusting for drainage area or other variables. Because neither of these methods rely on direct measurement of flow in the stream in question, their error is unknown and they are inaccurate compared to direct measurement. They nonetheless provide valuable input in evaluating flood magnitude and both are considered in the following section.

For this project, we also analyzed the measured spot measurements of flow taken during spring snowmelt to estimate flood frequency. Though the analysis performed was not a standardized procedure, it nonetheless provides further weight and certainty to our estimates of peak flow magnitude and recurrence.

2.4.2.2 Regression and Extrapolation

Hydro Science (1995) used a number of different techniques to make estimates of Q2 for a restoration project near the confluence of Angora Creek with the Upper Truckee River. These included extrapolation from the Saxon Creek and Trout Creek basins, which have long periods of record. Regional regression equations were also used to develop an estimate of 20-25 cfs for the Q2 (flood that recurs, on average, every two years). Although this location is about one mile downstream of the project area and has a larger drainage area, it likely has a Q2 flow similar to the project area. This is due to the fact that the Q2 is in essence the snowmelt peak. The watershed between the location for this project and the location of Hydro Science (1995) estimates is low in elevation and would have already melted prior to the peak snowmelt runoff.

We also used the U.S. Geological Survey regional regression equation (U.S. Geological Survey 1977) to estimate flood flows for this study. This equation is based on watershed area, mean elevation, and mean precipitation. Using the downstream end of the current project area for watershed area, we calculated a Q2 estimate of 30 cfs. For a restoration project a few hundred feet downstream of the project area, Graham Mathews Associates (2003) estimated a Q2 of 53 cfs using the same equation. The large difference is not explained by the slightly larger watershed area at the downstream restoration site. It is more likely due to differences in estimation of mean basin precipitation; we used 34 inches in recognition of the asymmetry in the watershed topography (see Figure 2.5, hypsiometric graph) and that the watershed is locally on the lee side of the mountain. These extrapolation and regional regression analyses suggest that the Q2 is approximately 25 cfs (+/- 5 cfs).

2.4.2.3 Analysis of Available

Flow Measurements

Annual peak flows can only reliably be measured where streamflow is continuously monitored, since the probability of taking a streamflow measurement during the instantaneous annual peak would be extremely unlikely.

The only available annual peak flow observation comes from the continuous record made by GMA in 2002 (Figure 2.5).

The peak flow in 2002 was 19.4 cfs. While this data has the value of being directly measured, it only represents a single year, and 2002 was relatively dry.

The Forest Service data set contains a number of spot observations from additional years. Because the Q2 discharge is highly associated with the snowmelt hydrograph, high flow observations during the snowmelt period can be used to augment the single peak

Table 2.2. Spot observations of flow exceeding 17 cfs in the project area during the snowmelt period.

Date Time Streamflow (cfs

May 16, 1996 1100 44 May 19, 1997 1430 31.4 April 18, 2011 1430 29.4

March 24, 1998 1015 26.0 March 16, 2011 1230 23.8

April 4, 2011 Not recorded 23.8

February 14, 2000 1300 23.3

June 19, 2011 1830 21.5 May 14, 1996 1445 21.3 May 3, 1993 1500 19.0 May 25, 1999 1100 18.8 May 23, 1998 1705 18.0 April 22, 1997 1200 17.6 June 2, 1999 1010 17.6 May 6, 1996 0930 17.4 May 8, 2000 1515 17.0 May 11, 1993 1615 17.0 observation and assist in estimating Q2. To analyze the Forest Service spot observations, we first collected all observations that were made during snowmelt and exceeded a threshold of 17 cfs (Table 2.2).

There are two unknown factors in the use of these data for peak flow estimates. The first is whether the observation occurred during the daily peak, since the diurnal fluctuation in flows can exceed 50 percent. Given the relatively small size of the basin and predominant east aspects, Angora Creek probably peaks in the late afternoon or early evening. The recorded peak flow during 2002 occurred at 1515 hours which indicates that measurements taken in late afternoon reasonably approximate the maxima for the day. A number of these measurements were made in the morning or early afternoon before the daily peak and therefore underestimate peak flow. The second is whether the recorded flow represents the annual peak—the probability of this is very low. A final consideration is that only the 2011 observations were made post-fire. All of the other observations are representative of the pre-fire melt regime, which was likely lower magnitude because of higher forest cover. Due to these factors, the flows in the table, a number of which occur within the same water year, are all biased estimates of annual peaks on the low side.

Although the spot observations are negatively biased, they can nonetheless be used to evaluate magnitude of the Q2 flow. To do this, an annual series of the highest observed measurement was created (Table 2.3). Again, these measurements are spot observations rather than measured annual peaks, with the exception of 19.4 cfs in 2002 from the GMA data.

The median value of this annual series, which is the Q2 in an annual peak series, is 19 cfs. Since most of the observations in the table are known to have significant low-side bias, it is probably reasonable to expect that the true Q2 should be 20-30 percent higher, or in the range of 23-25 cfs. Obviously, this is a crude estimate of Q2, because of the short data record and known bias, but it nonetheless is based on actual measurements of flow.

It correlates well with estimates of Q2 from regression and extrapolation analyses, indicating a Q2 of 25 cfs.

2.4.2.4 Estimates of Magnitude of More

Frequently Occurring Floods

Estimation of flood magnitude with less than a two year recurrence interval is valuable for design purposes.

However, regional regression equations are not available for these smaller floods. Furthermore, the measured flow data available are not sufficient to make reasonable estimates. Note, for example, that there is a very large gap between the third and fourth largest annual measured highest flows (Table 2.3).

Flows in this range are typically used to estimate 1.5 year recurrence interval.

Table 2.3. Annual series of spot observation maxima in the project area (2002 is a measured annual peak).

Water Year Streamflow (cfs

1991 5.4 1992 4.3 1993 17.0 1994 3.81 1996 44 1997 31.4 1998 26.0 1999 18.8 2000 23.3 2002 19.4 2011 29.4

We therefore used a different technique to estimate the magnitude of more frequently occurring floods. The flow record for the Upper Truckee River at Meyers, a station with a long period of record, was analyzed to determine the relationship between the magnitude of the Q2, Q1.5, and

Q1.1 flows. Although the Upper Truckee River is a much larger watershed, we assume that the relationship between the magnitudes of these different discharges for this watershed will be relatively similar to that of Angora Creek due to regional similarities in snowmelt patterns. Given the estimate of the Angora Creek Q2 derived above, the Upper Truckee relationships can thus be used to derive estimates of more frequent floods.

Table 2.4. Estimates of discharge for frequently occurring floods based on scaling from Q2 using Upper Truckee River data as a reference.

Flow Recurrence

Interval

Ratio of Qx/Q2, Upper Truckee

River

Estimated Angora Creek Flow (Q2=25 cfs) Q1.5 0.70 17.5

Q1.25 0.49 12.25 Q1.1 0.36 9.0

Table 2.4 shows the results of this analysis. The second column shows the ratio between the discharges for smaller floods (Q1.5, etc.) to the discharge for Q2. This ratio was applied to the estimated Q2 for Angora Creek to derive the estimates for smaller floods shown in Table 2.4

2.4.3 Flow Duration

Flow duration is an assessment of the number of days, on average, that a candidate design flow is exceeded in a given year. For restoration design, we are particularly interested in duration of overbank flow. This is typically done by examining the snowmelt hydrograph and directly computing the duration of exceedance for any given discharge. With a number of years of continuous flow measurement, an estimate of average yearly flow exceedance can be obtained.

However, only the 2002 hydrograph is available, and that year was dry, with an annual peak flow of 19.4 cfs. The annual peak did not exceed our estimate of Q2, although it did exceed our estimates of flow for more frequently occurring floods. The number of days in 2002 that flow exceeded our estimates for floods of various recurrence intervals is shown in Table 2.5. An important consideration when examining this table is that it is not clear whether this hydrograph is based on daily peaks or daily mean flows. If it is mean daily flows, flow likely exceeded the discharge in question on more days than noted due to diurnal variability in flow during snowmelt, because days in which the mean discharge did not exceed the threshold may have had peaks which did. Nonetheless, the available data only represents a drier than average year.

Table 2.5. Two estimates of the number of days in an average year flows exceeded the discharge for floods of various recurrence intervals. Estimates using the 2002 hydrograph include only one dry year, while the estimates for the UTR are based on over 40 years of record.

Flow Recurrence

Interval

Days Exceeded, 2002 GMA hydrograph

Days Exceeded, UTR Above Meyers

Gage

Q2 0 1 Q1.5 1 8 Q1.25 4 14 Q1.1 16 26

To augment this information, we turned again to flow data for the Upper Truckee River, assuming that characteristics of flow duration likely approximate those of Angora Creek even though the basin is larger. Examination of mean daily discharge data over the period of record showed that mean daily discharge will exceed Q2 on the average about 1 day per year. Additional results are shown in Table 2.5. The estimated number of days that mean daily discharge exceeds a given discharge is much higher than just using the 2002 data alone, which is expected given that 2002 was a dry year.

2.4.4 Low and Base Flows

Based upon the observations by the USFS and the continuous data collected in WY2002 by GMA (2003), the creek is perennial in the project reach in most if not all years. The 2002 water year started out as an above normal year, with wet months in November and December, but January and February were very dry, and the spring was overall quite dry, so that streamflow in the late summer and fall probably approached that of a very dry year. The lowest recorded flow that year was 0.04 cfs.

In addition, to the continuous flow measurement during the dry year of 2002, the Forest Service spot observations also occurred during low flow periods. Sporadic data collection between 1991 and 2007 resulted in a total of 34 streamflow measurements during the period of July to October. There were occasional occurrences of two and sometimes three measurements taken on the same day. Table 2.6 gives the mean monthly flow based on the available data.

There are no recorded observations of zero flow within the project reach. Flows in August and September represent the seasonal minimum. At Lake Tahoe, the lowest flow is typically during late August or early September. Prior to that, the snowmelt recession supports flows in July and after mid- September evapotranspiration losses diminish and there is usually some recovery of flow which is well under way by late October.

From November through February, winter baseflow is steady and persistent and, based on Forest Service observations, is estimated to be between 0.5- 2.0 cfs.

Table 2.6. Mean monthly flow for July through October based on USFS spot discharge observations, 1991-2007.

Month Number of Observations

Mean Flow (cfs)

July 12 2.72 August 6 0.38

September 11 0.54 October 5 0.68

2.5 Ground Water Hydrology

Throughout the project area, the valley floor is extremely wet. Standing water is common in the valley bottom early in the season, and persists in many locations through the summer. Saturation in soil pits during surveys conducted between July 8 and 14, 2010 never exceeded 1.3 ft below the ground surface (Figure 2.6); saturation in most pits was less than one foot from the surface.

The high groundwater table throughout the project area is at least partly due to the recent Angora fire, which burned much of the upper watershed, substantially reducing evapotranspiration. Members of our project team familiar with the site over a number of years, as well as USFS personnel, have noted that the area appears considerably wetter post-fire.

However, several factors indicate that the project site has been very wet in the past, prior to the fire. Vegetation communities throughout the project area are indicative of a high water table, especially in the northwest corner of the site, a dense sedge meadow. A spring on the hillside near the south east edge of the site supports a vegetation community similar to a fen. Finally, peats, mucks, and moderately decomposed plant materials are common over the valley floor.

These materials are the strongest indication of a persistent high, cold, water table present in the project area prior to the recent fire.

The wet meadow character of the valley floor, with its uniformly high water table, is dependent on two principle factors:

• inputs of groundwater moving laterally downslope over long distances off the valley sides into the valley floor sediments.

• Limited ability of delivered groundwater to migrate downslope through the valley bottom sediments because of a diminished hydraulic gradient and valley bottom materials which have restricted hydraulic conductivity.

2.5.1 Groundwater Inputs to the Valley Bottom

The watershed produces large amounts of runoff, far in excess of what can be lost through evapotranspiration within the meadow. Although the majority of this runoff is delivered through the channel system, the overall channel density within the watershed is relatively low which indicates that precipitation and snowmelt readily infiltrate. The upland soils within the watershed are dominantly Jabu, Meeks, and Tallac series. All of these soils have either a duripan or fragipan at depth which limits further percolation and results in water moving via subsurface lateral flow. Flow rates are sufficiently slow and distances sufficiently great that there is typically some groundwater being delivered to the meadow year-round. Even without these pedogenic features, we would expect the same process to occur over lithic contacts where they occur.

The recent Angora fire has, in our opinion, vastly increased groundwater discharging into the meadow. This is particularly evident on the northwest side of the valley where the existing marshy area appears to have enlarged and gotten considerably wetter. Over a span of decades, as the upland forest recovers, the amount of groundwater inflow will diminish, but not stop. Again, the presence of mucks and peats within the valley bottom indicates that high water table conditions are not just associated with the recent fire, although the accretion of organic materials may be somewhat dependent on a cyclical fire regime to periodically remove forest cover and increase groundwater delivery to the meadow.

For the water table to remain near the ground surface, inflow rates to the valley bottom must match or exceed losses from the sum of evapotranspiration plus discharge into the stream channel plus export via subsurface flow routes. Subsurface flow rates on the valley sides are likely to be at least an order of magnitude greater than rates within the meadow because of the far steeper slopes and coarser materials. However, the valley slope within the project area is relatively high, approximately 1-2%, and there are numerous analogs of valley bottoms in similar climatic and topographic settings which do not exhibit such a high water table with valley slopes that steep. Therefore, the dominant control in this system must be restricted hydraulic conductivity within the meadow alluvium.

2.5.2 Meadow Soils and Groundwater

The soils borings reveal two basic factors which restrict hydraulic conductivity within valley bottom meadow soils. First, the “sandy loam with clay” horizon, although not ubiquitous, is present over large portions of the meadow, particularly at the lower end. While this horizon does transmit water through it, its transmissivity is likely low relative to high inflow rates and it acts to restrict groundwater discharge from the meadow, at least during some times of the year. We observed groundwater entering the incised channel on top of the sandy loam with clay horizon when the water surface was well below where the groundwater entered (Figure 2.7), showing that, at these locations, the horizon was influencing groundwater flow.

Second, the highly heterogeneous nature of the soils in the meadow limits lateral continuity in groundwater flow. While there are numerous loamy and sandy horizons within the meadow which have very high hydraulic conductivities, they were created by local channel events such erosion or channel avulsion, and therefore tend to be spatially discrete and discontinuous.

Intervening zones of materials with much lower hydraulic conductivity, i.e., the silty mucks and peats and the sandy loam with clay horizon, thus act as the effective hydraulic control on the system.

2.5.3 Groundwater and the Stream

High rates of subsurface inflows to the valley bottom in the project area, in combination with high horizontal and vertical heterogeneity in subsurface sediments, cause subsurface inflows to rise to the surface during the snowmelt season and into the summer, in spite of the relatively steep valley slope. In this environment, the elevation of water in the channel is not highly correlated with the elevation of the water table in the adjacent floodplain. Even cursory observations confirm this condition; in the spring, water is clearly seen weeping into the channel through the banks or cascading over the top of the bank (Figure 2.7). Later in the year, although groundwater flow rates had declined and inputs of groundwater were not obvious in the streambanks, the water table in soil sampling pits was often higher than surface water in the adjacent channel.

2.6 Floodplain Topography

The floodplain in the project area is relatively steep, with an average slope of about 1% (Figure 2.8). Floodplain topography is irregular for an alluvial surface. The floodplain generally tilts to the north in much of the project area, especially in the upper portion, and is fairly strongly convex.

Although slope generally decreases in a downstream direction, some steeper areas are found in the lower portion of the project area and some relatively flatter areas can be found in the upper portion.

For about 300 feet upstream of the road crossing, floodplain slope is significantly lower and the channel is confined within a vegetated bowl-shaped depression about five feet below the surrounding meadow surface. This feature, which generally parallels the spur access road off Lake Tahoe Blvd., was probably constructed along with the road. Its’ function was likely to collect water during higher flows and funnel it under the road in one location.

The floodplain exhibits high diversity in microtopography. Low hummocks, open and closed small basins and small abandoned and active channels are abundant. Some of these channels were large enough to be effectively surveyed, and show up on Figure 2.8. Most, however, were very small and few were continuous for more than one hundred feet.

2.7 Vegetation Community

Field vegetation surveys were conducted in the project area in the summer of 2010. Surveys also were conducted in two similar meadows for comparison; about 300 feet downstream of the project area (REF01), and approximately ½ mile downstream of the project area (REF02), both on Angora Creek. Wetland plant community composition and cover data were evaluated using ocular cover estimates. The primary objectives of the vegetation surveys were to determine plant community composition, general status, and potential trends. Main findings of this survey are described and discussed in this section.

The meadow in the project area is a wet meadow as described by USDA Forest Service Pacific Southwest Rangeland and Planning Guide (Revised 12-2007). Although fens exist nearby, no plants particular to fens were noted in the surveys of the valley bottom meadow. The hummocky nature of the meadow in the project area is a key characteristic which contributes to higher diversity in the project area compared to the other sites.

There was a general trend in seral status of plants from the project area downstream through the other two sites examined. Nearly half of the plants identified in the project area were of early seral status, and later seral plants (especially shrubs) were lacking. The next meadow downstream was classified as mid-seral, and the most downstream late-seral. The trend in seral stage mirrors a trend in slope, suggesting that the higher slope in the project area probably is the basis for more regular fluvial disturbance (channel dynamics, sediment deposition).

Clearly, the recent fire is also influencing seral status in the project area, as the other two areas did not burn. Burned stumps throughout the meadow suggest that it burned vigorously. Total plant cover, while relatively high at 70%, is lower than at either of the downstream sites (REF01=96%, REF02=162%). Anecdotal observations of the plant community by persons familiar with the site since the fire typically focus on a very vigorous response in terms of revegetation, probably both due to a fire release response (reduced canopy cover, competition) and to increases in groundwater flow and elevation.

Extraordinarily high diversity and complexity in topography and hydrologic characteristics have resulted in a very complicated, diverse plant community within the project area. These factors make it very difficult to identify any trends in community characteristics. A relatively dry area occurred in a 50 foot wide strip along the lower 300 feet of the channel, but only on the southeast bank. Smaller drier patches occurred throughout the meadow.

2.8 Watershed Sediment Supply and Transport

Following the Angora Fire, there was widespread concern over the potential for high rates of sediment delivery to the channel from denuded slopes. A weir-like structure was constructed across the channel and adjacent floodplain just upstream of the Lake Tahoe Blvd. road crossing to collect excess sediment, protecting downstream areas.

To gain an understanding of the potential for delivery of large volumes of sediment to the stream, we performed an ocular survey of sediment sources and transport dynamics in the watershed upstream of the project area and downstream of Angora Lakes. Our objective was to identify areas of potential sediment supply, both natural or due to human disturbance, and evaluate the channel to identify reaches that were either sources of sediment, generally transporting sediment, or depositing sediment. Photos were taken at pertinent locations, along with notes describing the site (see Table 2.7 for notes, and Figure 2.9 for site locations).

The North Fork or mainstem of Angora Creek drains Angora Lakes. For the first quarter mile or so downstream of the lakes, the creek runs through a low relief landscape that is probably a terminal moraine at the end of the cirque containing the lakes. We did not survey this reach, as the low relief clearly provides little or no opportunity for significant erosion.

Exiting this landform, gradient of the channel increases significantly, and the creek is confined between the Angora Ridge moraine to the north and various glacial deposits to the south.

There is evidence of fairly high sediment supply in this reach, both from channel banks and from localized, small toe-of-slope erosional areas in the adjacent hillslopes (Sites 12 and 13, photos, Figure 2.9). We saw no evidence of larger-scale sediment sources such as debris flows or hillslope failures.

There is evidence of substantial sediment transport in the channel; we noted large sediment deposits behind debris accumulations, often two to three feet deep (Site 10, photo, Figure 2.9).

This sediment is generally supplied locally, where the channel is eroding the constraining morainal deposits in discrete locations where is comes in contact with them as at Site 12. These features are less than three feet in height, and confined to a few locations.

Just downstream of Site 14 on the mainstem, the channel becomes highly depositional. It is braided and areas between the channels are heavily colonized by riparian shrubs such as alder.

Sediment transport capacity is clearly low, such that most sediment generated upstream is deposited in this reach.

The South Fork, draining Seneca Pond and areas to the south and west, is located in terrain with less relief. Sediment availability is less than in the North Fork. A few minor, localized erosion areas were noted, generally in association with road crossings, but the amount of sediment generated at these sites was not significant in terms of channel response. No large-scale potential sediment sources from roads were found during our surveys. The South Fork drainage is supplying low rates of sediment to the system, as can be seen from the stream channel, which shows no sign of high sediment supply (Site 7, photo, Figure 2.9).

Results of our sediment survey indicate that sediment supply to the project area from the upper watershed is relatively low. General characteristics of the channel near the confluence between the North and South Forks reflects low sediment supply (Site 16, photo, Figure 2.9).

Streambanks are stable at this location, there are no large instream bars, even though woody debris is very abundant. We observed no obvious indications of a fire-related pulse of sediment working its way through the channel system, and low turbidity levels were observed during prolonged high flows that occurred in May, 2011.

Table 2.7. Sites visited during the watershed sediment survey, with notes describing each site (the location of each site is shown on Figure 2.9).

Location Notes on Potential Sediment Sources, Channel Sediment Dynamics

Photo 1 Small drainage, some minor erosion potential, sediment delivery low.

Photo 2

Downstream of bridge crossing. Stream has enough energy to erode gravel/cobble from adjacent moraine, but no evidence of high sediment transport rates—tree downstream does not have large sediment wedge associated with it.

Photo 3 Upstream of culvert, same road crossing as in photo 2. Streambed may be slightly aggraded at this location, but relatively minor. No high bars associated with backwater from culvert.

Photo 4 Road crossing, low erosion.

Photo 5 Road crossing, no erosion or evidence of high rates of sediment transport in channel.

Photo 6 Same location as in Photo 5, looking upstream at dense growth along channel.

Low potential for erosion.

Photo 7 Main Angora drainage (North Fork). Looking at sediment stored behind woody debris. Little evidence of high sediment load.

Photo 8 Same location as in Photo 7, slightly upstream. Extensive woody debris, but no evidence of excess sediment.

Photo 9 Small rill erosion off Angora lateral moraine, only location in survey where a concentration of flow was noted.

Photo 10

Main Angora channel (North Fork). Note stability and sediment storage due to wood. This surface has numerous other channels, which suggests that avulsion is relatively common. Moderate sediment supply combined with available wood likely provides avulsion potential. Note slug of sediment upper center of photo.

Photo 11 Sediment slug in Photo 10.

Photo 12

Channel just north of the main…