Geophysical Characteristics of Rivers and Streams

Geologic structure affects water chemistry, patterns of hydrology and drainage formation, local substrate, slope, and longitudinal profiles of the streambed. As such, the geomorphic processes that have been occurring over 200 million years strongly influence the structure and function of todays stream biota. The long period of mountain formation in the southern Appalachians produced extremely complex bedrock patterns (Hack 1969, Isphording and Fitzpatrick 1992). Variation in rock types and their resistance to weathering and erosion have important influences on local relief, landscape forms, and water chemistry (Wallace and others 1992):

• Granites and other light-colored, course-grained, crystalline rocks. Fairly resistant to erosion and water penetration, often form areas of steep slopes. These rocks erode primarily by mechanical chipping.
• Mica schists. Often occupy areas of low relief in the Blue Ridge. They are generally very susceptible to chemical and mechanical weathering.
• Sandstone and quartzites. Resistant to both chemical and mechanical weathering. These rocks often form ridges in the Appalachian Valley, form the underlying rock of plateau tops in the Cumberlands, and are typically found in areas of steep slopes. Streams flowing through areas of sandstone and quartzite are lined with cobbles and boulder substrate.
• Carbonate rocks. Include limestone and dolomite, which react with weak acids in rainwater, soil, or groundwater. The limestone, or dolomite, is dissolved and components are transported in solution. Some limestones contain silica and calcium sulfates, which result in higher relief than belts of pure limestone.
• Shale. A nonresistant rock, which is easily penetrated by water. It decomposes readily into smaller fragments, and often forms areas of relatively low relief and thin soils.

Channels are often bedrock-controlled in the headwaters, while alluvial channels may occur in valley rivers.

Channel slopes of small headwater streams are typically inversely related to their flow rates, or discharge. Since discharge and stream length are directly related, channel slopes tend to be inversely proportional to stream lengths (Wallace and others 1992).