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E. HEALTH OF RIVER CHANNEL AND LAKE WATER WITH INCREASED FLOWS
This project will develop a set of recommendations to minimize further sediment and salt loading to Walker Lake and degradation to the lower Walker River under increased water flows. These recommendations will be made available to land and water managers to assess potential impacts resulting from variations in flow, water quality and channel geometry on the transport of sediments and on the flow capacity of the Walker River.
Researchers:
From DRI: Ken Adams (PI), Division of Earth and Ecosystem Sciences; Rosemary Carroll (Co-I) Division of Hydrologic Sciences (DHS); Li Chen (Co-I), DHS
From
UNR: Keith Dennett (PI), Department of Civil and Environmental Engineering (CEE); Eric Marchand (Co-PI), CEE; Kyle House (Co-I), Nevada Bureau of Mines and Geology, and Joanna Redwine (graduate student)
Updates:
Previous Year(s): 2007
2008: January | February | March | April | May | June | July | August | September
October | November
January 2008:
High resolution LIDAR data for the lower Walker River are now available. This topographic data was collected in May/June of 2005 and has 2 m horizontal resolution. Air photo mapping is ongoing and is being incorporated into the GIS database. The combination of detailed elevation data sets from 1995, 1997, and 2005 and rectified aerial imagery from 1938, 1954, 1974, 1997, 2005, and 2006 now allows a detailed view of how the river has responded to the lowering of Walker Lake (see Figure 1).
Field surveying to gather river cross section data and water and sediment samples is still continuing, but at a much slower pace because of weather conditions. Surveying along the west fork of the river and the combined Walker River outside the reservation has been completed. A few long reaches remain to be surveyed along the east fork of the river.
Mapping of the lower Walker River and how it has changed is ongoing, with the goal of documenting how stream bank and bed erosion has proceeded through time. The combination of detailed elevation data sets from 1995, 1997, and 2005 and rectified aerial imagery from 1938, 1954, 1974, 1997, 2005, and 2006 is providing a detailed view of how the river has responded to the lowering of Walker Lake.
Field surveying along the east fork of the Walker River to gather river cross section data is progressing. Monthly collection and analysis of water samples on the east fork, west fork, and combined river is also continuing.
Field surveying along the east fork of the Walker River to gather river cross section data is progressing. Monthly collection and analysis of water samples on the east fork, west fork, and combined river is also continuing.
We are now able to measure the amount of sediment eroded from banks of the lower Walker River for discrete periods of time, which provides important information on how the river is responding to various magnitudes of flow.
In the spring of 2005, approximately 654,000 metric tons of sediment was eroded from the banks of the lower Walker River (LWR) during a one month period when flows reached a peak of about 1400 cubic feet per second (cfs). This unusually high sediment yield attests to the ongoing instability of the LWR, as it continues to adjust to falling lake levels.
Figure 1. Combined curves showing the spring runoff hydrograph for 2005 and the associated lake-level rise with respect to when the LIDAR topographic data and imagery were collected (vertical bars). Note that the LIDAR data was collected on the rising limb and the imagery was collected on the falling limb of the hydrograph.
Figure 2. Walker River delta on June 28, 2005 just after peak discharge for that year (~1400 cfs) had occurred. The red line signifies the boundaries of the channel and delta in late May of that year and the pink shaded area indicates the areas that were eroded in that one month span.
Ongoing studies along the lower Walker River demonstrate that historic incision and bank instability not only affects the ecology of the system but also may pose a hazard to bridges and other infrastructure in the Schurz area.
In previous months, we have reported on how unstable the lower Walker River is and how much erosion occurs with each passing runoff season. The location of the current instability, however, is not random. Most of the erosion occurs in the lowermost few kilometers of the river and is likely a direct response to lake-level lowering. An additional factor that might be contributing to high erosion rates is the relative lack of vegetation along the banks of the lowermost river.
Moving upstream, there is a deep trench that is the net result of historic incision in response to Walker Lake lowering from its historic highstand in 1868. The headcut at the upstream end of this trench is located at the siphon, an irrigation structure that crosses the river just downstream from Schurz. The siphon consists of a broad diameter steel pipe that crosses the bed of the river but is no longer being used because it has been bypassed by a bridge that connects the irrigation ditches on either side of the river.
Field surveying along the east fork of the Walker River to gather river cross section data is almost complete. Monthly collection and analysis of water samples on the east fork, west fork, and combined river is also continuing. Testing of sediment samples to determine erosion rates is underway.
Work has been progressing on all facets of this project including sediment transport modeling, field sampling, and the GIS database. We now have collected aerial photographs from 13 different years beginning in 1938 and continuing to the present for our GIS database. Our challenge now is to calibrate the sediment transport model to the observations of erosion and bank retreat.
Field surveying along the east fork of the Walker River to gather river cross section data is complete except for approximately 12 miles where access has not been permitted by landowners. Collection of samples for water quality analyses will occur in mid-June. Testing of sediment samples to determine erosion rates will be completed in late June.
The sediment transport model for the lower Walker River is being used to evaluate the amount of sediment transport occurring in the 100-year, 50-year, 20-year, 10-year, 2-year, and mean annual floods. We are also evaluating the amount of sediment that is predicted to have moved using historic flows. Of particular interest is the spring 2005 runoff event when approximately 654,000 metric tons of sediment was eroded from the banks of the lower Walker River (LWR) during a one-month period when flows reached a peak of about 1400 cubic feet per second (cfs). Modeling the sediment moved in this runoff event will be an important test for the model.
For the second year in a row, there was a poor runoff season for Walker Lake. This spring there were two pulses of water that reached the lake. The first was in late May and early June when the river was flowing at an average of 91 cubic feet per second (cfs) for 9 days. The second period of river flow began on June 14 and continued through June 26 with an average flow of about 45 cfs. In an average year, discharge begins to ramp up in April, reaches around 500 cfs in late May, and continues at this relatively high rate until mid-June. By late June, flows are typically back down to about 300 cfs. The net result of this year’s low inflow is that the lake will likely fall another 3-4 feet by the fall.
Field surveying along the east fork of the Walker River to gather river cross section data is complete except for approximately 12 miles where access has not been permitted by landowners. A hydraulic computer model of the upper Walker River including the east fork, west fork and combined river has been developed. The computer model is being used to identify allowable flow rates in order to minimize erosion and sediment transport in the Walker River. Water quality sampling has been completed, as has testing of sediment samples in the laboratory to determine erosion rates.
Results from GIS analyses, sediment samples, and sediment transport modeling are now being compiled and synthesized in order to understand in detail how the lower Walker River has responded over the last 100 years to fluctuating flows and falling lake levels. Briefly, relatively low flows in the river combined with a steadily falling lake level have led to severe incision, which in turn has flushed millions of tons of fine sediment and salt into the lake. The net result of these effects has left the river in a highly degraded condition and has reduced the water quality of Walker Lake. The erosional history of the lower Walker River will be part of the report for this project.
Results from this study are being compiled into a report that will also present recommendations for the conveyance of water in the Walker River in order to minimize erosion and sediment transport, and how to minimize further degradation of the lower river and reduce sediment and salt loading to the lake.
There are no updates this month.
Draft final reports were completed and submitted for review.
Final reports were completed on schedule and are in review.
Draft final reports have been completed and are in the process of being reviewed and revised.