ACWI May 1999 Attachment 18 p. 1

To: Tim Smith
U.S. Geological Survey
From: Emery T. Cleaves
Re: ACWI - New Business
Date: April 23, 1999

     Small watersheds commonly comprise fifty to seventy-five percent of the landscape. Because of the continuing growth of urban - suburban areas into agricultural and forested lands it is especially important to understand how small watersheds are being affected and what management measures may minimize the impact of change and otherwise preserve and protect those watersheds of importance to us.

     The primary controls on watersheds are climate and geologic framework. Climate provides the energy and water to watershed systems, and the geologic framework provides the material which is shaped into a variety of land forms by the energy and water supplied by climate. Water is the key element. Water and its movements, (as ground water and surface water and atmospheric water) and its interactions with the rocks, soils and plants provide the setting for terrestrial and aquatic living resources. Changes to the physical and botanical setting result in major changes to other living resources.

     Understanding small watersheds is fundamental to stream and watershed management and restoration. The stream integrates the various processes active in the watershed, yet has little affect on soil and vegetation and their interactions with climate and geologic framework outside of the flood plain. The watershed catches precipitation and partitions it into surface runoff and ground water flow. The rate at which water reaches the stream depends upon intensity of the storm event, soil, vegetation, geologic framework and land use. The capacity of soil to absorb, store and transmit water depends upon weatherability of the bedrock and soil development. Plant communities depend upon soil water storage capacity which in turn is related to lithology. The geologic framework strongly influences watershed morphology, slope stability and steepness as well as erosion, leaching and potential productivity. The substrata of the stream is strongly related to lithology and degree of dissection of the landscape. Consequently, it is of great value to assess the processes operating in a small watershed and to identify the resources at risk as human development alters the energy and material fluxes through and within them.