Habitat Complexity

Habitat complexity is variability in the physical structure of the stream channel and is important to fish in developing and maintain pools, spawning habitat, and feeding habitats over a full range of flows and through all seasons.  The example displayed above is taken from a project implemented on private property for the purpose of improving habitat complexity for spring Chinook and steelhead.  The pre-project photo (upper left) of the restoration site visualizes a wide and shallow channel with swift water and no pools or cover for adult or juvenile Chinook or steelhead to live in.   High winter or spring flows would damage or destroy spawning redds and winter flows would wash juvenile fish downstream.   A stream channel in this condition provides poor spawning and rearing habitat, and will not feed very many fish.  In July 2015, as part of a larger project, one log jam and a single log were placed within the channel (red arrows upper right image post-project) which increased channel bed roughness leading to the development of gravel bars (blue arrows).  The formation of the bars helped to form a number of deep pools (red arrows) and by 2017 more than six new flow paths developed (yellow arrows).

Spring Chinook and steelhead juvenile and adult fish benefit from the results of habitat complexity when changes in stream velocity sorts gravels that provide spawning habitat (yellow arrow above).  The formation of deep pools provide habitat for both adult and juvenile salmon to rest out of main river current and hide from predators.  The interstitial spaces within the log jam where the deep pool is along the log jams edge also provides protected areas for juvenile Chinook and steelhead to rest and feed during the winter.  Over time, the additional flow paths and the floodplain connectivity will aid in supporting and developing more resilient riparian forests and provides more useable space (surface area) for juvenile fish to spread out and feed.