Evaluation of fish and habitat characteristics near the Muir Beach Community service district’s water supply well in Redwood Creek, Marin co, California

Fong, Darren; Manning, David; Becker, Ben

The National Park Service conducted a multiyear study to determine the influence of well operations on instream flow conditions for fish within the Redwood Creek watershed, Marin County, California. The ultimate goal of the study was to identify mitigation measures that would ameliorate the effects of pumping operations on aquatic life and to identify scientifically based thresholds for monitoring and action. The results indicated a short lag time between pump activity and response in the creek during dry years and dry season conditions. Comparison of pumping activity and stream water surface levels showed connectivity between groundwater conditions at the well and adjacent creek water surface elevations. Under dry conditions, normal pumping rates can temporarily lower water surface elevations and sometimes below streambed surface. Dry creek conditions and disconnection of pool habitats have occurred for nearly 50% of the years since 1991. These conditions have occurred when rainfall amounts during the rain season (November through April) are less than 33 inches. The disconnected pools and dry creek areas are downstream of the Muir Beach Community Services District (MBCSD) facility and can extend 2.4 km to the creek’s confluence with the Pacific Ocean. Isolated pools have resulted in significantly lower dissolved oxygen concentrations than pools with upstream riffles providing surface water inflow. Based on past data, extended weather conditions during the summer-fall low flow period can result in major changes in water surface elevations, pool connectivity and dissolved oxygen conditions. No major differences in fish metrics can be detected sites upstream and downstream of the MBCSD facility during years with average or greater rainfall amounts during the rain season. However, densities, overall fish production, and diversity below the MBCSD facility are reduced during dry years. The few fish that manage to survive are generally plumper when compared to similar fish upstream of the MBCSD facility and to all sites during wetter years. Under dry year conditions, there is a potential for a 10 percent reduction in watershed fish production. The contribution of water withdrawal activities to this reduction is unknown. We provide several recommendations that would help reduce impacts on instream fishery resources or fill key data gaps. These recommendations include: initiation of actions to obtain “take” authorization for listed fish, reevaluation of current pumping schedule to protect natural streamflow patterns, changing the definition of “dry year” to 33 inches of rainfall during the rain season, and continued evaluation of means to reduce water consumption and/or increase water supply and storage.