Tuolumne River Report - U.S. Fish and Wildlife Service

LINKING PHYSICAL PROCESSES AND SALMON LIFE HISTORYCHAPTER 2Figure 2-32. Conceptual cross section through a spawning riffle to show the effects of flow variation on distributingspawning area to lateral margins of the channel.2.4.2.3. Hydrologic conditions andchinook life historyAdaptive responses to the hydrologic regime werecritical elements molding chinook life historystrategies. Not only did streamflow and floodsform and maintain instream habitat, but chinooksalmon also adapted to the natural variability inmagnitude, timing, duration, and frequency ofstreamflow events. Although only relatively smallportions of their lives are spent in the riverenvironment, these life stages are criticallyimportant to the population.We analyzed unregulated hydrograph componentsto examine historical flow patterns with significanceto chinook salmon life history adaptations.These conditions were then compared to thecurrent hydrologic conditions set by the FERCNDPP license amendment in the SettlementAgreement.Immigration and spawning. Chinook salmonimmigration and spawning occurred during fallbaseflows and fall storms of low magnitude andshort duration. Unimpaired average fall baseflowsranged from 332 cfs to 542 cfs during CriticallyDry and Extremely Wet water years, respectively,and frequently exceeded 1,000 cfs during wetterwater year types. Variability in streamflow duringthe spawning season, combined with diversechannel topography, provided variation in depthand velocity that was an important mechanism todistribute spawning within different channellocations. This mechanism maximized availablespawning habitat (Figure 2-32), and probablyreduced scour and superimposition mortality onincubating eggs.Minimum streamflows during the spawningseason are now determined by the FSA flowschedule (Table 2-5), which sets steady, nonfluctuatingflow releases, and does not incorporatehistorical streamflow variability. The effectsof flow regulation on the hydrograph duringspawning can be seen by comparing representativeunimpaired and regulated water years (Figure 2-33). Regulation of fall baseflows at unnaturallylow streamflows may produce the undesirableconsequence of limiting spawning to the center ofthe channel as opposed to margin habitat (Figure2-32), encouraging salmon to construct theirredds on top of pre-existing redds (redd superimposition),and increasing the vulnerability of eggpockets to scour during moderate or large floods.59