Evaluation of alternative streamflows to improve aquatic and riparian habitat in the Walker River Basin, Nevada

Edmund Andrews, Susan Mortenson, Chad Gourley

ABSTRACT
This report describes an investigation of streamflows and sediment transport in the Walker River Basin of Nevada and California. The natural regime of streamflows has been altered by storage in three major reservoirs and many diversions both large and small to the extent that total basin outflows to Walker Lake have been reduced by approximately two-thirds. The objectives of this study are to develop and evaluate alternative streamflow regimes for the Walker River that will improve the quality and quantity of aquatic and riparian habitat. Achieving these objectives will involve: defining the relationship between streamflows and sediment transport, calculating daily and annual fluxes of various particle sizes with prevailing flow regimes, and evaluating alternative flow regimes. Suspended and bedload sediment transport rates were sampled frequently in 2011 at eight USGS streamflow gaging stations. The relative portions of washload and bed-material load transported in the Walker River were found to be highly unusual. Typically, washload (i.e. silt and clay) represents approximately 80% of the total sediment load, whereas sand and gravel represent approximately 20%. In the Walker River Basin, washload represents only 15-33% of the total sediment load, whereas sand and gravel represent 67-85%. The estimated mean annual sediment yield of all sediment sizes is approximately 4.3 tons/km2-yr, which is quite low compared to similar drainage basins across the western U.S. Accordingly, the current erosion rate, and thus, the supply of sediment, does not appear to be sufficient to produce the abundance of sand-size bed-material commonly found in reaches of the Walker River. Instead, reduced peak flows and channelization of the river channel have caused accumulation of sand within the channel.

At all gage records considered, a majority of all sediment transport over a period of years is carried by relatively large, though not extreme, streamflows. A number of possible flow regimes have been evaluated and prioritized using the analysis of sediment transport magnitude and frequency presented to determine how and to what extent such flow regimes could improve aquatic and riparian habitat throughout the Walker River Basin. Aquatic and riparian habitat would be improved by reducing the abundance of sand-sized bed material, increasing the extent of available gravel bed material, increasing pool depth, and increasing the frequency of overbank inundation. The additional quantity of sediment transported by adding a given increment of runoff volume will be greater when it is applied to a larger streamflow than a smaller one. Furthermore, overbank sand deposition will be maximized by augmenting the largest sustained streamflows of the year. Even relatively small streamflow increases would provide appreciable immediate improvement in the aquatic and riparian habitat of the Walker River.

Keywords: Sediment transport, Streamflow augmentation, Flow regime, Walker Lake
Citation: Andrews, E.D., Mortenson, S.G., and Gourley, C.R., 2015, Evaluation of alternative streamflows to improve aquatic and riparian habitat in the Walker River Basin, Nevada. Journal of the Nevada Water Resources Association, Winter 2015, p. 1 – 37. Copyright 2015 Nevada Water Resources Association.

Investigation of methylmercury concentrations in irrigation drains and impacts on receiving waters, Nevada, USA

Mae Sexauer Gustin, Christianna Peterson, Rebekka Fine, Travis Lyman, Musheng Alishahi, Matthieu B. Miller, Susan Donaldson

Abstract
Understanding the impact of agricultural practices in areas with legacy mining contamination is important across the globe. To investigate this, mercury (Hg) concentrations in water and sediment in the Carson, Truckee, and Walker watersheds in Nevada, USA, were monitored in irrigation ditches seasonally during 2011 and 2012. In addition, concentrations in source and receiving waters upstream and downstream of ditch input were monitored. Legacy mining Hg contamination associated with amalgamation of gold is present in two of the watersheds (Carson and Truckee). Natural enrichment may be the Hg source in the Walker River. To assess the impact of legacy Hg contamination, Carson and Truckee River study areas were chosen upstream and downstream of regions of Hg contamination. For the Walker River, potential impacts were investigated only downstream of potential sources of Hg contamination. Total Hg (THg) in water and bed sediment in the rivers upstream of irrigation drains were similar to other non-contaminated rivers around the world (1 to 11 ng L-1 and 3 to 160 ng g-1; respectively), whereas at downriver locations concentrations were generally higher than reported elsewhere (2 to 760 ng L-1 and 26 to 30,000 ng g-1; respectively). Similarly, methylmercury (MeHg) water and sediment concentrations were lowest upriver (82 to 1,140 pg L-1 and < 0.1 to 42 ng g-1; respectively), and higher downriver (200 to 13,400 pg L-1 and < 1 to 760 ng g-1; respectively). Highest THg and MeHg concentrations were primarily observed in association with particle-based transport during spring corresponding with seasonal runoff and irrigation drain flushing. Results indicate that water managers and irrigators should manage systems to limit the use of water derived from spring runoff and drain flushing to reduce Hg loading to fields and wetlands. In addition, having ditches and drains flow more slowly, be free of vegetation, and be shallow and wide will promote demethylation of MeHg produced.

Keywords: Mercury, Sediment, Nitrogen, Phosphorus
Citation: Gustin, M.S., Peterson, C., Fine, R., Lyman, T., Alishahi, M., and Miller, M.B., 2015, Investigation of methylmercury concentrations in irrigation drains and impacts on receiving waters, Nevada, USA. Journal of the Nevada Water Resources Association, Winter 2015, p. 38 – 60. Copyright 2015 Nevada Water Resources Association.