Shoreline Erosion and Geomorphology of Canyon Ferry Lake

Abstract

Canyon Ferry Lake, east of Helena in Broadwater County, Montana was formed in 1954 when Canyon Ferry Dam was constructed across the Missouri River. Lakeshore residents have reported excessive erosion during recent years. In 2012, the Bureau of Reclamation produced a management report with recommendations for erosion mitigation, but no studies have been done on Canyon Ferry to determine relative erosion risk and site-specific factors driving shoreline processes. This study is an analysis of shoreline erosion around Canyon Ferry Lake. The first component of this work included the classification and mapping of shoreline morphotypes lake wide. This allowed for the identification of areas subject to higher erosion risk based upon the driving forces that resulted in differing shoreline morphology. The Steep Cliff and Sloped collectively accounted for 46.9% of shoreline length lake wide, highlighting the need for erosion mitigation. The shoreline morphotypes were then mapped in relation to geology or rock type, and also beach aspect to determine whether these two factors are influencing the presence of given morphotypes, and by extension, whether these factors influence relative erosion risk at Canyon Ferry Lake. The strongest relationship was between Steep Cliff and Tertiary sedimentary rocks. Relationships between morphotype and aspect (wave energy) were inconclusive.