See also: Addressing Sediment as a Limiting Factor

Flow Regimes as a Limiting Factor

While the specific flow requirements of salmonids vary for each species, local populations of salmon and steelhead have additionally evolved the necessary physiological and behavioral characteristics for them to survive the dynamic flows encountered during each phase of their life history at a specific time of the year. Flow regimes are one of the most important drivers of habitat structure at micro-, reach, and riverscape scales. During critical stages while eggs incubate, young fry forage or drift, and adults struggle to return to spawn, the relationship between flow volume and velocity demonstrates the importance of a variable flow pattern over daily, seasonal, and annual time periods that is critical for the long-term persistence of salmonid populations. A critical review by Bjornn and Reiser (1991) summarized a majority of the known literature concerning flow regimes and their relationship to salmonid habitat.

Predation, fishing, and disease are pressures facing most local salmon and steelhead populations, yet the potential difficulty of finding preferred spawning habitat is also a significant limiting factor. The volume and velocity of water within a watershed is often a cue for initiating spawning migration, and is the fluid boundary which limits redd construction. Culverts present difficulties for adult and juvenile salmonids migrating upstream, though the extent to which this is a barrier to passage often depends upon a particular species leaping ability. Flows can be problematic in streams and rivers even when the watercourse does not pass through culverts. A shift in the peak flows may strand salmonids in the wrong portion of a watershed preventing them from reaching spawning habitatand potentially subjecting them to predation while waiting for additional flows to pass further into the watershed or forcing them to spawn in lower reaches where egg survival is limited. Although there appears to be little observational data about minimum depths necessary for passage, a minimum depth of 12 cm for trout, 18 cm for steelhead and coho salmon, and 24cm for Chinook salmon are considered necessary for passage (Bjornn and Reiser 1991). FishXing (http://www.stream.fs.fed.us/fishxing/resources.html) is an awesome resource, in both English and Spanish, which allows the user to evaluate and design fish passable culverts.

The timing and velocity of flows during spawning are critical characteristics of spawning reaches, and poor flow conditions can limit the survival of eggs and alevins. In locations like Butte Creek, redd imposition may limit the survival of earlier spawning Spring Run Chinook salmon, which are an important genetic component of this distinct stock of California Chinook salmon. With adequate management of flows released from upstream hydroelectric facilities, the quantity of spawning gravels available to Butte Creek Spring Run Chinook could be increased and egg survival potentially increased. Most salmonids need water depths of at least 15cm, though this is also variable and dependent on spawner density and possible upwelling and hyporeic flows. Smith (1973) described the depth and velocity characteristics of 1,170 redds of 10 species of salmonids, including all the species in California. Regardless, flows need to be sufficient to not limit the velocity of oxygen-rich water through the stream’s hyporeic zone. Kondolf (2000) suggested a unified approach for assessing the impacts of sediment during the critical spawning, incubation, and emergence stages and incorporated a life-stage specific evaluation of gravel and flow requirements.  

Flow and water depth are also critical determinants of rearing habitats for fry, parr, and adult residents and together comprise the amount of habitat available to salmonids in streams. High flows can be responsible for side-and off-channel habitat formation, which provide critical micro-habitat for rearing. Flow regime is one of the factors dictating salmonids emigration from freshwater to the ocean. While long-term flow increases are likely necessary to support a steady rate of out-migration of populations far from the ocean, even small short-term increases can be an important stimulus for coho and Chinook salmon. Beechie et al. (1994) determined that 73% of summer habitat loss and 91% of winter habitat losses for a coastal Washington population of coho salmon were associated with hydromodifications associated with agriculture and urban lands. NMFS (PCSRF 2005) ranked degraded freshwater habitats and flows as moderate limiting factors to coho salmon recovery.  The return of a dynamic, natural flow regime will be critical to minimize the threats of these limiting factors. Roni et al (2002) reviewed strategies for restoration and suggested a tiered watershed approach that first restored connectivity, then returned natural hydrologic and geologic variability and processes, before finally focusing on instream restoration that is necessary for salmonid recovery.  

Bjornn, T.C. and Reiser, D.W. 1991. Habitat Requirements of salmonids in streams. Pages 83-138 in W.R. Meehan, editor. Influences of forest and rangeland management on salmonid fishes and their habitat. Special Publication 19. American Fisheries Society, Bethesda, MD.

Beechie, T.J., Beamer, E. and Wasserman, L. 1994. Estimating coho salmon rearing habitat and smolt production losses in a large river basin, and implication for habitat restoration. North American Journal of Fisheries Management 14:797-811.

Kondolf, G.M. 2000. Assessing salmonid spawning gravel quality. Transactions of the American Fisheries Society 129: 262-281.

PCSRF (Pacific Coast Salmonid Restoration Fund). 2005. 2005 Report to Congress, 2000-2004.

Roni, P, Beechie, T.J., Bilby, R.E., Leonetti, F.E., Pollock, M.M., and Pess, G.R. 2002. A review of stream restoration techniques and a hierarchical strategy for prioritizing restoration in Pacific Northwest Watersheds. North American Journal of Fisheries Management 22:1-20

Smith, A.K. 1973. Development and application of spawning velocity and depth criteria for Oregon salmonids. Transactions of the American Fisheries Society 102:310-316.