Why does the draft policy have a minimum bypass flow criteria? How was the minimum bypass flow criteria developed?
The minimum bypass flow is the minimum instantaneous instream flow rate that is adequate for fish spawning and passage. In terms of water diversion, it is the minimum instantaneous flow rate of water that must be moving past the point of diversion before water may be diverted. A minimum bypass flow requirement prevents water diversions during periods when stream flows are at or below the flows needed for spawning and passage. The draft policy’s minimum bypass flow criteria was developed utilizing basin size (drainage area) and hydrology (mean annual flow), focusing on stream flows needed for protecting spawning habitat. Data on instream flows needed for steelhead spawning were obtained from a study by C. H. Swift (1976) and from 13 study sites within the policy area. Both data sets represented the lowest flow at which maximum spawning habitat availability occurred for steelhead. The data sets were used to develop an equation for the minimum bypass flow that is a function of drainage ar
