Variability of shoreline erosion and accretion within a beach compartment of Flathead Lake, Montana

Abstract

We measured shoreline reconfiguration and variability in shoreline retreat in Flathead Lake, which is a large glacial lake regulated by a hydroelectric dam at its natural outlet. Wave‐induced erosion rates ranged from 0.5 to 2.5 m yr −1 at the northshore study site where fetch, and hence wave energy, was maximal. Shoreline retreat was not solely related to storm intensity or volumetric changes in nearshore bars. Three erosive processes were involved in the measured variability of shoreline retreat: undercutting, endstripping, and overwash. Wave scour on an exposed bank caused undercutting and endstripping, while overwash resulted when water was forced over the top of the shoreline. Shoreline morphology and vegetation determined the type of erosion process and the rate of shoreline retreat. Shoreline retreat was offset by localized and dramatic accretion caused by sediment entrapment by drift logs bordering the shoreline. The eroding shoreline and nearshore bed were sediment sources for the accretion. Drift logs naturally protected the shoreline from direct wave attack and stimulated sediment accretion, providing new recruitment area for riparian vegetation.