Harold Jones | Geology Reports

USGS2026

Methods for the computation of streamflow statistics for selected streamgages in or near Nevada through water year 2021

Streamflow statistics are used by Federal, State, and local agencies to protect and manage the water resources of Nevada. Peak-streamflow, low-streamflow, streamflow-duration, mean, minimum, and maximum streamflow statistics were computed for 310 streamgages in Nevada and surrounding States through water year 2021 (the period from October 1, 2020, to September 30, 2021) by the U.S. Geological Survey. Trend analyses of a subset of 54 active streamgages with at least 30 years of data that were minimally affected by human activities indicated that most sites had “somewhat likely downward” or “likely downward” trends for peak, mean, and low streamflow. Statistically significant (probability value less than 0.05) negative trends were associated with peak streamflow at 12 percent of sites, annual mean streamflow at 31 percent of sites, and annual 7-day average low streamflow at 28 percent of sites. Statistically significant positive trends were associated with peak streamflow at 2 percent of sites, annual mean streamflow at 3 percent of sites, and annual 7-day average low streamflow at 9 percent of sites. Comparisons with previously computed statistics determined that changes in computational methods, the length of the streamgage record, and instance of wetter than average periods were the most influential on the computed statistics.

Arizona, California, Idaho, Nevada, Oregon, Utah↗

USGS2026

Flood-inundation extents and hydraulic characteristics of Carson Slough, Ash Meadows National Wildlife Refuge, Nevada

Mud Lake Dam, a rock and earth embankment structure, crosses an ephemeral stream that once flowed naturally into Carson Slough, the largest wetland complex in southern Nevada and a critical habitat for several threatened and endangered species. The dam was breached in the 1990s, rerouting stormwater into a new channel. The Bureau of Land Management and U.S. Fish and Wildlife Service are considering possible dam modifications to establish streamflow back to the natural channel. The U.S. Geological Survey, in collaboration with the Bureau of Land Management and U.S. Fish and Wildlife Service, developed hydrologic and hydraulic models to assess how peak-flow breach expansion and possible dam modifications could affect flooding in Carson Slough. A precipitation-runoff model was developed to estimate 6-hour and 24-hour flood streamflows for 10-, 25-, 50-, and 100-year floods, which were used as inputs to a hydraulic model simulating three scenarios: 2023 conditions, peak-flow breach expansion, and possible dam modifications. Model outputs included flood-inundation extents and hydraulic characteristics such as water depth and velocity. Under 2023 conditions, streamflow enters the dam breach and follows the breach channel to the west into Carson Slough, inundating Soda Spring. Only under the highest flood scenario (24-hour duration, 100-year rainfall recurrence interval) did streamflow also enter the natural channel directly downstream from the dam. Under possible dam modification conditions, streamflow remained solely in the natural channel and flowed southwest into Carson Slough. Under peak-flow breach expansion, streamflow entered the breach and natural channels and into Carson Slough. Compared to 2023 conditions, the total inundated extent for peak-flow breach expansion increased by 7–11 percent, whereas possible dam modification conditions decreased by 4–7 percent. Inundation extents remained largely similar throughout most of the study area, and the primary exception was in the 1-mile segment directly downstream from the dam.

Nevada↗