Response of Delta Vegetation to Water Level Changes in a Regulated Mountain Lake, Washington State, USA

Water level fluctuations are well known to structure wetland vegetation along lakeshores, but the hydroperiod components that drive plant community change are often not differentiated. This study characterized river delta plant communities and examined the distribution and hydroperiods of dominant species in response to a changing, hypervariable lake water regime. At three intervals over an 18 year period we mapped plant communties and developed hydroperiod graphs of dominant plant species at comparable points along their elevation distribution. These low diversity community types (sedge, willow, and cottonwood/alder dominated) generally shifted upslope as lake water levels increased. Hydroperiod graphs showed that the dominant sedge species (Carex vesicaria) moved upslope enough to maintain a similar hydroperiod as lake water levels increased, but the dominant shrub species (Salix spp.) did not. The lower sedge boundary was best correlated to late summer water levels, indicating that inundation duration and growing season length was likely controlling the lower limit of sedge vegetation. Salix spp. distribution was likely controlled primarily by maximum depth and secondarily by inundation duration. Hydroperiod graphs provide a simple way of relating key hydrologic variables to the distribution of wetland vegetation, and to predict vegetation response to future reservoir water levels.