VARIABILITY OF SHORELINE EROSION AND ACCRETION WITHIN A BEACH COMPARTMENT OF FLATHEAD LAKE, MONTANA

We measured shoreline reconfiguration and variability in shoreline ret reat in Flathead Lake, which is a large glacial lake regulated by a hy droelectric dam at its natural outlet. Wave-induced erosion rates rang ed from 0.5 to 2.5 m yr-1 at the northshore study site where fetch, an d hence wave energy, was maximal. Shoreline retreat was not solely rel ated to storm intensity or volumetric changes in nearshore bars. Three erosive processes were involved in the measured variability of shorel ine retreat: undercutting, endstripping, and overwash. Wave scour on a n exposed bank caused undercutting and endstripping, while overwash re sulted when water was forced over the top of the shoreline. Shoreline morphology and vegetation determined the type of erosion process and t he rate of shoreline retreat. Shoreline retreat was offset by localize d and dramatic accretion caused by sediment entrapment by drift logs b ordering the shoreline. The eroding shoreline and nearshore bed were s ediment sources for the accretion. Drift logs naturally protected the shoreline from direct wave attack and stimulated sediment accretion, p roviding new recruitment area for riparian vegetation.