Effects of Artificial Habitat Structures on Fluvial Geomorphology and Trout Distribution

Artificial habitat structures are widely used to restore ecosystem functions to streams that have been impaired by human activity.  Goals of these structures are often to increase abundance of valued fisheries by increasing habitat complexity and providing cover where natural woody material is lacking.   Despite wide use, evaluations of effectiveness have shown variable results for fish abundance, and few studies have evaluated the implications of artificial structures on fluvial geomorphology.  The North Branch of the Au Sable River (Michigan) is a blue-ribbon trout stream, but the effects of historical logging are still apparent; the relatively young riparian corridor contributes little shade or woody material recruitment. Further, this river is dominated by groundwater, leading to low stream power and potentially different responses than in other stream types. The goal of this study is to evaluate the impact of structures on fluvial geomorphology and trout distribution in this stream.  Individual structures often created local habitat conditions favorable to trout use, but often entrain fine sediments downstream of the structure, leading to unfavorable conditions.  Rather than installing structures where convenient, practitioners should use restoration actions that take into consideration fluvial processes and the tradeoffs that occur between local and reach level channel forms.