The Smith River basin includes more than 3100 miles of stream channels (California Department of Fish and Game 1980) including an estimated 176 miles of anadromous fish habitat and 114 miles of resident fish habitat (McCain et al. 1995). The river network has identifiable habitat units defined by geomorphology. These units are present at various scales and form a convenient hierarchy for conceptualizing the watershed. Geomorphic units range from river basin, subbasin, reach, and small habitat unit such as pool and riffle. The Smith River basin is composed of four distinct subbasins (Table 20, Figure 2 will be added). Several conventions are recommended for describing and classifying smaller stream units. Classification at the scale of stream reaches can follow methods developed by Rosgen (1985). McCain et al. (1990) describe an appropriate method for classifying stream habitat units in terms of channel features. For most management purposes, watersheds are advantageous planning units because they are stable and their boundaries are easily located. For understanding fish populations, watersheds and habitat units are indispensable.

Geologic structure has a dominant influence on stream characteristics. With the exception of alluvial channels on the coastal plain, the Smith River system consists primarily of steep narrow bedrock-controlled channels formed as streams have cut through resistant bedrock. There are also areas of less resistant rock where the terrain is relatively less steep and streams develop broader channels with gentler gradients. Due to the widespread presence of resistant geology, stream channels of the Smith River resist alteration due human activities and natural upheaval. Therefore channel morphology is less altered in the Smith River than in the Mad, Eel, and Mattole Rivers which flow through more highly erodable areas.

The lower Smith River subbasin extends from the mouth of the South Fork to the ocean and includes the estuary. The mainstem is much less restricted by bedrock than the other parts of the river. The Smith River flood plain is about a half mile wide near Fort Dick and widens as it flows west across the coastal plain. Mill Creek and Rowdy Creek are the two largest tributaries below the South Fork. Moderate slopes are found on the lower reaches of these two tributaries.

The North, South, and Middle Forks of the Smith River appear to be superimposed streams. This means the streams have cut down into underlying bedrock while maintaining a pre-existing meander pattern (McCain et al. 1995). Some of the major tributaries, including Siskiyou Fork, Craigs Creek, Hurdygurdy Creek, and Goose Creek, also have entrenched meander patterns in their lower sections. Almost all of the upstream tributaries flow through narrow bedrock valleys with the exception of a few reaches near Big Flat and Gasquet. Many tributaries are not smoothly graded at their confluence with the mainstem. The presence of nick points or alluvial fans indicates recent geologic activity. In general, the mainstems are downcutting at a higher rate than the tributaries.

The North Fork subbasin consists of the North Fork and its major tributary, Diamond Creek. The channel is steep and V-shaped. The North Fork includes a gorge that cuts through peridotite. Compared to the other subbasins, the North Fork subbasin has experienced relatively little change due to human activities and retains most of its natural character. There are fewer roads and timber harvest units than in the other subbasins. The North Fork provides high quality aquatic habitat (McCain et al. 1995).

The Middle Fork subbasin includes Patrick Creek and the Siskiyou Fork as major tributaries. The channel gradient is steep in the headwaters and moderate in downstream areas. Most streams in the Middle Fork subbasin flow through V-shaped canyons. Upstream from the confluence with the South Fork, the Middle Fork includes a gorge that cuts through metamorphosed basalt. Slopes are less steep upstream from Gasquet due to less resistant serpentine bedrock. In this subbasin, riparian and stream habitats have been heavily altered due to flood events, timber harvest in riparian areas, road-related erosion, and mining.

The South Fork subbasin is the largest subbasin and has many tributaries. The channel is steep and V-shaped except in the lower five miles. The gorge on the lower South Fork cuts through metamorphosed basalt (greenstone). Slopes are less steep upstream from the South Fork gorge due to less resistant serpentine bedrock. Moderate slopes are also found along Jones and Goose Creeks in the South Fork subbasin.

Upstream from the confluence of the South Fork and the Middle Fork, stream catchments are smaller and therefore respond more rapidly to rainfall. As a result, the flow regime in the subbasin mainstems and their tributaries is characterized by rapid changes in discharge, velocity, and stream hydraulics. Because downstream areas have delayed response to precipitation, these larger streams tend to experience less rapid changes in discharge and velocity compared to upstream areas (McCain et al. 1995). Downstream areas also have lower gradients and therefore lower water velocities.

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