a Encompasses runs, glides, and rapids as commonly used in fisheries literature b Pool spacing is expressed in channel widths a Encompasses runs, glides, and rapids as commonly used in fisheries literature b Pool spacing is expressed in channel widths substrates: bedrock, alluvium, and colluvium. Bedrock channels lack alluvial deposits and have high transport capacity relative to sediment supply. Colluvial channels typically are small headwater streams that receive sediments from debris flows and hillslopes and have weak capacity for sediment transport. Alluvial channels are further subdivided into five categories that occur along the river continuum owing to threshold changes in the processes summarized above.

In an idealized long profile from upper hillslopes to lowland river, these occur in longitudinal sequence (Figure 1.4). Cascade channels occur on steep slopes, are confined by valley walls and have a substrate that typically consists of cobbles and boulders. Pools commonly are small and spaced less than one channel width apart. Cascade channels retain larger clasts but rapidly transport smaller sediments to lower-gradient channels. In step-pool channels, longitudinal steps are formed by larger clasts, resulting in discrete pools with a spacing of one to four channel widths. Gradients are steep, width to depth ratios are low, and valley wall confinement is evident. At moderate to high gradients and in relatively straight channels, the channel type known as plane bed develops. It usually includes a somewhat featureless combination of riffles, runs, and rapids. The substrate includes gravel, cobbles and small boulders, and may be armored or not. An armored substrate indicates a transport capacity greater than sediment supply, whereas a substrate that is not armored indicates a balance between transport capacity and sediment supply (Dietrich et al. 1989). Thus the plane-bed stream is transitional between supply-limited upper reaches and transport-limited lower reaches (Montgomery and Buffington 1997). Pool-riffle channels with pools spaced at five to seven channel widths occur at moderate gradients. They represent a shift to a more transport-limited system, although this varies with degree of armoring. Dune-ripple channels typically are low-gradient, sand-bed systems experiencing significant sediment mobility under most flow conditions.

The five alluvial channel types just described are referred to as free-formed, meaning they result from the interaction of sediment supply and transport capacity. In forested mountain streams,

LW can force channel morphologies where they might not otherwise occur (Figure 3.6). LW can cause local scour and flow divergence, and may result in sediment accumulation in otherwise bedrock streams. A forced pool-riffle system is one where most pools are the result of LW, and a forced step-pool structure can result from regularly spaced, channel-spanning LW.

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