Stream channels: the link between forests and fishes

The hydraulic characteristics of flow through channels arean important component of fish habitat. Salmonids have evolved instream systems in which water velocity and flow depth vary spatiallywithin the watershed and temporally on a daily, seasonal, and annualbasis. Flow requirements vary during different phases of the freshwaterlife cycle of salmonids: free passage is necessary during migration ofadults; clean and stable gravel beds ensure successful incubation ofeggs; and adequate velocity and depth of flow provide space forsummer rearing and overwintering. The life cycles of salmonid specieshave adapted to the temporal variations in flow conditions by timing thephases of the life cycle to take advantage of the seasonal dischargecharacteristics. Spatial variability enhances species diversity bycreating a variety of habitats within stream reaches; these arepartitioned among individual species and age groups having differenttolerances for velocity, depth, and cover conditions.
Channel morphology is determined largely by sediment and waterinput to the channels, and is formed during storm events when flow isgreat enough to transport the coarse sediments lining the channel bed.The resulting channel shape consists of a seauence of recognizable
units known as riffles, pools, and boulder cascades. Water flowing downthe stream is forced continually to adjust its velocity and depth inresponse to the changing channel shape: flow is shallow and fast inriffles, and slow and deep in pools. Large obstructions such as woodydebris, boulders, and bedrock outcrops alter channel width, increasingthe variation in velocity and depth in the vicinity of the obstructionand anchoring the position of pools. Discharge also varies through time,creating additional variations of hydraulic conditions.
Forest management can affect channel morphology by changing theamount of sediment or water contributed to the streams, thus disruptingthe balance of sediment input and removal. Excessive input of coarsesediments from landslides can smooth the channel gradient by fillingpools. Removing large woody debris from channels reduces sedimentstorage and eliminates the local hydraulic variability associated with theobstruction. Loss of habitat diversity by either mechanism may reduceor change the fish species found in a stream reach. If the changesresult in decreased space, populations may also decrease. Strategies tominimize the effects of land management on channel morphology and fishhabitat should include practices that minimize increases in coarse sediment input, and that preserve the morphologic complexity of thechannel.