Lake Level Regulation and Shoreline Erosion on Flathead Lake, Montana: A Response to the Redistribution of Annual Wave Energy
Keywords:Wave energy, shoreline erosion, sea level, lakes, reservoirs
Concern regarding increased coastal erosion has heightened amid growing acceptance of global warming and associated sea-level rise. Attention has focused on the absolute elevation of sea-level rise, whereas shoreline erosion may result more from the redistribution of wave energy over the duration of the sea level rise. This study examines shoreline erosion in Flathead Lake, Montana, resulting from a redistribution of annual wave energy due to five decades of regulated lake levels by a dam at the lake's outlet. The annual rise in lake level is held for an extended duration at a regulated "full pool" level which is approximately 3 m above its natural base elevation. The total time for a complete cycle of lake level change also has been increased from 100 days to over 300 days. Alteration of the natural lake level fluctuation has resulted in a transfer of annual wave energy from base elevations corresponding to the pre-dam nearshore shelf, to elevations corresponding to the limnetic foreshore environment, thereby inducing lake-wide erosion. The most extensive erosion has occurred along the low-lying dissipative north shore of the lake. The redistribution of annual wave energy due to regulated lake level fluctuation is the main factor contributing to that erosion. The redistribution (If annual incident wave energy is examined in terms of an increase in the duration of the complete natural lake-level cycle, a reduced range in lake level fluctuation, and an elevated full pool lake level. Both the duration of lake level cycle and range in water level fluctuation affect the distribution of incident wave energy and were found to be more important to shoreline erosion than an absolute elevated water level. An alternative regulation scheme incorporating concerns for both hydropower production and lake recreation is proposed and examined in terms of reducing erosion. Altering the regulated lake level fluctuation by changing the timing and increasing the rate of lake level drawdown, would result in a significant reduction in the amount of annual wave energy that reaches the eroding full pool shoreline, thereby reducing the potential for lake-wide shoreline erosion.