Stillwater developed a rapid assessment protocol and modeling tool to understand the overall water demand required to support steelhead throughout San Luis Obispo County (County). The County’s Master Water Report (MWR) included an estimate of Environmental Water Demand (EWD), defined as the instream flow needs to support federally threatened steelhead. However, the EWD did not provide estimates for specific seasons, subwatersheds or creeks, or take into account seasonal fluctuations in flow, or support real-time compliance monitoring. Biologists from Stillwater Sciences are gathering existing data and implementing a process for estimating useful and appropriate environmental instream flow requirements for the County.
EWD was defined in relation to steelhead life history requirements during the two most flow-sensitive periods for minimum flows, namely the spring period and the summer period. All available hydrologic and physical terrain data were evaluated to assess patterns of instream flows and stream morphological characteristics, such as channel gradient, channel width, and geologic terrain. A field-based instream flow assessment was conducted, and a predictive model was developed to estimate EWD for all watersheds in the County.
Overall, it appears that spring flows are sufficient to provide steelhead habitat in many watersheds under existing conditions. It also appears that although summer flows are not sufficient in many County watersheds under existing conditions, based on channel morphology even relatively low flows (e.g., <0.5 cfs) during summer allow steelhead to persist in County watersheds. However, estimates of EWD are minimum values to maintain aquatic systems and should not be interpreted as “enough” water to support long-term, sustainable steelhead populations or the complex ecosystem in which they live.