Lower Mill Creek Watershed Conjunctive Use Project: Tehama County, California

In 1920, the Tehama County Superior Court of the State of California adjudicated entitlements to all Mill Creek flow below 203 cubic feet per second (cfs). As such, water right holders on Mill Creek legally divert a significant portion of the surface water flow for agricultural beneficial use.

During certain times of the year, especially during dry or critically dry years, the agricultural demand for surface water can reduce Mill Creek flow and expose in-stream barriers to fish migration. Fishery experts recognize Mill Creek as a high priority stream for the protection and enhancement of Chinook salmon spawning habitat.

Mill Creek surface water diverters currently participate in a long-term cooperative management plan to help provide sufficient flow for fish migration while also maintaining irrigation supplies and the recognition of surface water rights. As part of these efforts, Mill Creek water users participate in water lease and groundwater exchange programs designed to increase in-stream flow during critical spring and fall fish migration periods.

Along with other entities, the Department of Water Resources (DWR) plans to establish methods of monitoring and studying fish passage, assessing agricultural water use efficiency, and examining the potential for additional use of groundwater as opposed to use of surface water diversions in the Lower Mill Creek watershed.

This report provides a background of the geology and hydrogeology in the Lower Mill Creek watershed area, a detailed discussion of the groundwater resources, and an overview of the potential for conjunctive use of surface water and groundwater resources. It also provides recommendations for additional groundwater monitoring and potentially favorable locations for production well installation associated with possible future conjunctive use programs.