The Long-Term Stability and Morphologic Influence of the Use of Instream structures in Channel-Restoration Design

Abstract

Widespread use of instream structures in the United States occurred as early as the 1930s under the direction of the Civilian Conservation Corp. Specific designs of instream structures were well developed in the 1930s with prototype versions of most currently employed designs. The Civilian Conservation Corp and U.S. Forest Service helped to standardize the designs in the 1930s and subsequent decades, and promote their use throughout the country. Many modern designs of instream structures show little or no variation in materials, construction methods and placement specifications relative to the 1930s prototypes. Consequently, surviving instream structures from projects in the last seventy years provide an opportunity to assess the long-term stability and morphologic impact of these devices. Field studies in the northeastern U.S. show that decay of structures can create site conditions that adversely impact aquatic and riparian habitat. Loss of streamside trees and reduced bank cover were both noted with certain designs of instream structures. Additional literature-based findings show that instream structures require expensive yearly maintenance to insure continued function of the devices as designed. These results show that the use of instream structures do not always provide a cost-effective longterm approach to channel-restoration design. Furthermore, the detrimental long-term impacts of certain designs of instream structure may reduce the amount of specific habitat areas stream managers try to enhance.