Stream flow is critical to sustaining riverine plants and animals, many of which have adapted to historic flow patterns. These species include the foothill yellow-legged frog, whose reproduction is timed with the annual spring snowmelt, and Pacific salmon that migrate up the Sacramento and spawn in the creek. Mill Creek, a tributary of the Sacramento River, hosts one of the highest elevation salmon-spawning habitats in California and is one of the few streams that still support threatened spring-run Chinook salmon. However, the stream routinely goes almost completely dry during irrigation season (June through mid-October) when landowners with senior water rights divert flows to irrigate nut orchards and cattle pasture. This research focused on the development of a spreadsheet model that calculates and identifies environmental-flow shortages based on seasonal diversion demands and water management scenarios, such as water exchanges, water-rights purchases and substituting groundwater for creek water as a useful first step in deciding how to address environmental water shortages in diverted creeks and rivers. This work was conducted at The UC Davis Center for Watershed Sciences in collaboration with The Nature Conservancy.

Graphic: Upper Diversion Dam at Mill Creek (Tehama County).