Corroborating the Protectiveness of Regional Instream Flow Criteria of the California North Coast Instream Flow Policy

The California State Water Resources Control Board adopted the Policy for Maintaining Instream Flows in Northern California Coastal Streams in 2010, establishing guidelines for evaluating the potential impacts of winter diversion projects on stream hydrology and biological resources, with emphasis on protecting ESA-listed steelhead trout.   Policy Elements include a diversion season, minimum bypass flow (MBF), and a maximum cumulative diversion rate (MCD).  Regional protectiveness criteria were developed for each element for the case when site-specific data and analyses were not available.  The MBF criterion relied on drainage area and mean annual flow to specify minimum steelhead upstream passage and spawning flows.  The MCD relied on universal geomorphic stream geometry relations to establish a diversion rate that would not adversely affect natural flow variability channel maintenance processes.  In 2013, we initiated a field study to further evaluate the protectiveness of the regional criteria.  Our new data and analyses corroborated the protective basis behind the regional MBF  and MCD flow rate criteria.  This talk presents the basis behind, and the new results supporting, the use of (i) a protective envelope-curve relation for as a regionally protective MBF criterion and (ii) fundamental geomorphic principles for specifying limits to diversion rates.