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Chapter 4 -- Watershed Processes and Aquatic Ecosystems (continued)

Fluvial Sediment Processes

Stream channel characteristics are strongly influenced by the rate of sediment input and the amount of stream power available to transport sediments. Sediments in the channel are either transported or deposited, depending mostly on stream gradient. Stream reaches with gradients less than 1.5 or 2% have less capacity to transport sediment and tend to store sediments in gravel beds and other deposits. Streams with steep gradients transport more sediment. In streams with intermediate steepness, both transport and deposition occur. Due to variability of annual flow, sediment transport varies widely from year to year.

Streams in the upper and lower areas of the Smith River network differ greatly in channel gradient and sediment regime. Channel gradients are steeper in the headwaters and decrease downstream. Because of the steep gradients, channels in upstream areas have more energy for transporting sediments. In these streams, alluvial deposits are found only in short low gradient sections. These streams are primarily controlled by bedrock.

Streams on the coastal plain, including the lower Smith River and sections of its tributaries, have lower gradients. From the confluence of the South and Middle Forks down to the Highway 199 bridge, channel gradient averages ten feet per mile. Below the Highway 101 bridge, channel gradient averages about three feet per mile. Tributaries of the lower Smith River, such as Mill and Rowdy Creeks, tend to have gentle channel gradients in their lower reaches. These streams store vast quantities of sediment and create a channel through the accumulated sediments. During high stream flow, the active channel of alluvial streams is subject to scouring and deposition, which changes its shape, elevation, and location within the valley. In addition, peak flows may create or reopen side channels such as along the backsides of point bars.

High precipitation in this region adds a large measure of energy to the streams allowing them to carry away large amounts of sediment. The steep terrain of the watershed further increases the capacity to transport sediment. Therefore, the streams above the confluence of the South and Middle Forks have a very high capacity to transport materials. Because sediment loads are transported downstream more rapidly, these stream habitats recover from large sediment inputs relatively quickly, e.g. ten to twenty years. The high capacity for moving sediment contributes to the generally high quality aquatic habitat throughout the Smith River system.

Stream terraces are formed where bedrock has resisted erosive forces or where materials are aggraded but are not completely removed by subsequent incision of the river. Smaller terraces in the steep upper watershed are often formed by bedrock that is planed off by the river. Stream terraces are also formed as streams cut through materials deposited in the channel by large landslides. Accumulations of sediment in the lower river have been sculpted into large alluvial terraces including the Fort Dick terrace on the southern edge of the flood plain. Other smaller alluvial terraces are found north of the Smith River and along Rowdy Creek (California Department of Water Resources 1987).

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