Minimum Flow Requirements. The minimum flow requirements prescribe the flows in the lower American River water to meet fishery needs throughout the entire water year. These minimum flow requirements include minimum release requirements (MRR) measured downstream of Nimbus Dam, and downstream flow requirements (250 cfs from January through mid-September and 500 cfs from mid- September through December) between Nimbus Dam and the mouth of the lower American River. The prescribed flows are minimums only and do not preclude Reclamation from making higher releases. The MRR varies from 800 to 2,000 cfs throughout the year in response to the hydrology of the Sacramento and American River basins and a set of prescriptive and discretionary adjustments. As such, the specified MRR is higher in wet years and lower in dry years. These adjustments are made in response to specific conditions related to the need for spawning flow progressions, fish protection, and reservoir water conservation. The resultant MRR varies throughout the season as shown in Table 1. Table 1. Seasonal Variation in the Minimum Release Requirement Time Period MRR Range (cfs) Index Relevance of Index October 800 to 1,500 Four Reservoir Index (FRI) Indicates the amount of upstream storage available during the fall and winter months November and December 800 to 2,000 FRI January and February 800 to 1,750 Sacramento River Index (SRI) Indicates current multi-basin water availability March through Labor Day 800 to 1,750 Folsom Inflow Index (IFII) Forecasts water availability for the American River Basin for the remainder of the current water year Post-Labor Day through September 800 to 1,500 IFII The FMS also includes exceptions to the MRR during extreme dry conditions, including: Conference Years: Occur when the projected March through November unimpaired inflow to Folsom Reservoir is less than 400,000 AF. A minimum flow of 190 cfs is required downstream of the H Street Bridge. Off-ramp Criteria: Triggered if Folsom Reservoir storage is forecasted to fall below 200,000 AF in the succeeding 12 months. In this case, downstream flow requirements rather than MRR become the minimum flow requirement throughout the lower American River.
