CONTRIBUTIONS OF GLACIAL MELTWATER TO STREAMFLOW IN THUNDER CREEK, NORTH CASCADES NATIONAL PARK, WASHINGTON: A MODELING STUDY

Glacial meltwater is a vital component to natural hydrologic systems and for hydroelectric and municipal purposes. A concern for water resources managers is that glaciers in the North Cascades have been slowly retreating. The glacial ice coverage of Thunder Creek watershed in the North Cascades National Park, the most heavily glaciated basin in the North Cascades, has dropped from approximately 22.5% to 12.8% since the Little Ice Age (LIA) maximum (c. 1850). Glacial melt-water contributions to Thunder Creek is of interest because the creek serves as a tributary to Diablo Reservoir, which is one of three reservoirs on the Skagit River maintained by Seattle City Light for hydroelectric power production. In this study, the Distributed Hydrology Soils Vegetation Model (DHSVM) is used to evaluate the effects of glacial retreat on discharge in Thunder Creek.

DHSVM is a physically based model that simulates a water and energy balance at the scale of a digital elevation model (DEM). Basin characteristics in DHSVM are defined by GIS maps of topography (DEM), watershed boundary, soil type, soil thickness, vegetation, and a flow network. The input meteorological requirements for DHSVM include time-series data representing air temperature, humidity, wind speed, incoming shortwave radiation, incoming longwave radiation and precipitation. These data were compiled from recent historical records of local weather stations except for longwave radiation, which was estimated. We calibrated DHSVM for water years 1998-2002 to annual discharge and monthly hydrographs measured at the USGS gauging station at Thunder Creek using 2-hour time steps and a 50-meter pixel size. DHSVM is used to calculate discharge at Thunder Creek with LIA glacial conditions and glacial conditions at 50, 100, 150, 300 and 500 years in the future based on current rates glacial retreat.

Preliminary simulation results using current meteorological data indicate an increase in annual discharge in Thunder Creek during the LIA and a significant decrease in annual discharge as the glaciers retreat in time.