Spring Snowmelt Recession in Rivers of the Western Sierra Nevada Mountains

TitleSpring Snowmelt Recession in Rivers of the Western Sierra Nevada Mountains
Publication TypeThesis
Year of Publication2011
AuthorsEpke, G.
Academic DepartmentHydrologic Sciences
DegreeMaster of Sciences
Number of Pages67
UniversityUniversity of California
Other NumbersUMI Number: 1502261
Keywordsrecession, rivers, Sierra, Snowmelt
AbstractThe hydrographs of rivers flowing from California’s Sierra Nevada mountains can be characterized by three distinct components; dry season baseflows, wet season storm pulses, and springtime snowmelt. The springtime snowmelt recession limb occurs as flows drop from snowmelt to summer baseflow. It is a consistent and predictable portion of the annual hydrograph and an important resource to both riverine ecosystems and California’s water supply, but reservoir and dam operations commonly eliminate this feature. Environmental flow allocations to promote healthy rivers are have started to include a snowmelt recession limb component, but little research has been conducted to quantify their form in the Sierra Nevada. This study fills this knowledge gap by describing the recession limb and its variability between water years and watersheds for unregulated flows. To do this, I chose eight watersheds without dams or significant hydrologic alterations, and, using historic discharge data, defined the recession period and calculated its magnitude, duration, timing, volume, and curvature. The recession shape, or rate of change, I modeled with an exponential decay curve in two different ways: one to describe the seasonal shape and the other for daily changes. I found that the recession limb typically lasts 75 days, from mid-May until August with differences in timing influenced by different watershed elevations. The magnitude of the discharge changes annually with different water year types, but the curvature is consistent across different water year types. Seasonally, this curvature is between -0.03 and -0.05 (std dev 0.007, NSME 0.64) whereas daily it decreases from 10 to 5 %. This research has important implications for the management of Sierra Nevada rivers in that it will allow for the inclusion of empirical quantitative criteria into the development of regulated flow regimes intended to better mimic natural ones.