HYDROLOGIC RESPONSE OF THE CROW WING WATERSHED, MINNESOTA TO MID-HOLOCENE CLIMATE CHANGE

Facies analysis of sediment cores collected from two lakes situated at opposite ends of the Crow Wing Watershed in north-central Minnesota reveals a dramatically different hydrologic response of these lakes to the mid Holocene warm period. Lake level low stands were identified within lake cores by the presence of sandy shoreline facies. Between approximately 3500 to 7700 14C years before present, upland Lake Mina experienced about a 14 meter drop in lake level while the lowland Moody lake only saw about a 4.5 meter drop. In the center of the watershed, Almendinger (1993) reported maximum water level declines in a series of lakes between 2.9 to 6 m during the mid Holocene. These data support the hypothesis that lakes within different portions of a watershed respond differently to the same climatic forcing due to the effects of groundwater hydrodynamics. Upland lakes will respond with higher amplitude and shorter frequency fluctuations to periodic climatic forcing than lakes located near the outflow point of a watershed. The effects of spatial variations in hydraulic conductivity further complicates the hydrologic effects of surface water and groundwater reservoirs to climate change. A three-dimensional numerical model was developed to assess the effects of aquifer hydrodynamics on paleoclimatic records from the Crow Wing Watershed using a coupled surface water/groundwater hydrologic model. The model was first calibrated to a 50-year historical record of average annual surface water discharge, monthly groundwater, and lake level fluctuations. The model was able to reproduce long-term historical records (1949-1999) of water table and lake level fluctuations across the watershed as well as stream discharge near the watershed outlet. The calibrated model was then used to reproduce paleo-groundwater and lake levels using climatic reconstructions based on pollen-transfer functions from Williams Lake located just outside the watershed. Computed declines in mid Holocene lake levels dropped between 8 to 18 meters across the watershed. Simulated streamflow near the outlet of the watershed at Pillager, MN decreased to 70% of modern average annual discharge levels after about 200 years. The area covered by wetlands was reduced by about 16%.