A fundamental difference between a canal and a natural stream is structure. Structure includes all the typical anomalies of natural streams that deflect the general downstream flow, such as bends, bars, bedrock knobs, boulders, landslide deposits, and large woody debris. This results in the storage of watershed products in the channel, and in a great heterogeneity in depth, velocity, stream gradient, and substrate conditions. In this paper, I will discuss these functions of structure in salmon habitat and some implications for restoration of habitat. Structure and Storage. Watershed products introduced to and stored in stream channels include water, sediment, and organic material. Watershed managers attempt to control the introduction of these products during management activities, but the amounts in channels depend mostly on storage compartments created by structure. In a general sense, storage of watershed products in a stream increases the overall productivity of the aquatic ecosystem. Storage of water in pools and backwaters greatly contributes to the minimum living space available during low flow in summer. Storage of sediment provides spawning gravel and also decreases the impact of large inputs of sediment by slowing and dispersing pulses of sediment as they move downstream (Swanson an
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