| Stream flow drives many physical and ecological processes in rivers that support freshwater ecosystems. Human activities like dam operations, water diversions, and flood control infrastructure together have fundamentally altered many streams. Water scarcity from increasing water demands and prolonged droughts has further stressed freshwater ecosystems, which in turn is prompting the development of new methods and tools for establishing environmental flows. This study developed a linear programming model for exploring the effects of water management alternatives on environmental flows in river systems that have minimal or no regulation from dam operations, but still have altered flow regimes due to surface water diversions. The model was applied to a case study on Mill Creek, a tributary of the Sacramento River in northern California's Tehama County, whose altered flow regime affects fall and spring fish migration to upstream spawning habitat. Test cases were used to examine how water management alternatives can improve environmental flow allocations while striving to meet agricultural water supply demands. These test cases included: 1) fish passage flows for California Central Valley Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss); 2) a 2.55 cms minimum instream flow (MIF); and 3) a sustainability boundary approach (SBA) with a flow target of 80% of natural flow. The model quantifies the effect of water management alternatives such as conjunctive use, water rights transfers, and water exchange agreements on instream environmental flow conditions. The model identified the last two weeks of October as a consistent period of shortage for fall fish passage flows during all water year types from critically dry to wet water years. Shortages to fish passage flows could be eliminated through the combined use of a water exchange agreement and conjunctive use wells. The MIF and SBA environmental flow cases both required acquisition of the largest water right holder in the system to decrease environmental shortages by over 80%. This modeling approach can be applied to other river systems as a decision-support tool for conservation organizations and government agencies to make more informed decisions regarding management of scarce water resources to maximize ecological function while minimizing impact on human uses of water. |
