Modeling Low Flows on the Cosumnes River

Abstract

As a part of a study examining alternatives for flow augmentation for fall run Chinook salmon migration on the Cosumnes River, California, two models were constructed to simulate channel flow combined with seepage losses. The first model is a numerical, one-dimensional diffusion wave approximation of the St. Venant equations coupled to a seepage routine that uses either a Green and Ampt infiltration routine or a head-based seepage routine that can handle seepage to or from the channel. The second model uses an analytical solution to the diffusion wave approximation of the St. Venant equations and also incorporates the Green and Ampt method to represent losses due to seepage for the simulation of flow over a dry riverbed. The two models are used to simulate low flow periods in the Michigan Bar to McConnell reach of the Cosumnes River in order to determine flows associated with a minimum depth necessary for fall run Chinook salmon migration. These computed flows were compared against historical data in order to determine volumes of flow augmentation for the time period October through December which corresponds to the spawning period of fall run Chinook salmon. Computed flow augmentation volumes were compared with available reservoir storage in the watershed in order to determine if such flow augmentation would be possible. Both models proved capable of simulating low flow periods on the Cosumnes River. During dry years, the computed flow augmentation volumes represent a significant percentage of the available reservoir storage, indicating that augmentation flows may not always be possible for the entire time period from this source alone.