It has been estimated that modern sediment loads of the rivers draining to the Atlantic Ocean may be four to five times greater than the prehistoric rates because of the effects of human activity. Even where human impact is large, however, it is possible to recognize several other independent variables that exert a major influence on long-term sediment yield. These variables can be grouped into three main classes: geologic, geomorphic, and climatic-vegetational. The major geologic influence on sediment yield is through lithology, or the composition and physical properties of rocks and their resistance to weathering and erosion. An easily weathered ...(100 of 32811 words)