DOCUMENTING CHANGE IN WATER LEVELS OF NORTHERN RANGE LAKES, YELLOWSTONE NATIONAL PARK

The small ponds and lakes of Yellowstone National Park’s Northern Range have been of interest for many years. This study focuses on five lakes occupying natural depressions in glacial sediments that overlie fractured bedrock ranging from Precambrian gneissic granite to the Eocene Absaroka Supergroup. Anecdotal observations over the last three decades indicate a pronounced, widespread recent drop in water levels and decreased surface areas. In some cases, lakes and ponds have completely dried up over the course of the summer. This study involved 1) mapping current water levels from mid-June to mid- July 2009 and high-water lines (breaks in slope, lichen lines, changes in vegetation) with a mapping-grade GPS unit and 2) quantitatively assessing as well as detecting change in lake surface areas using late August through early October aerial photographs (1954-2006). The water level response to widespread changes appears to differ depending on the hydrogeology of each water body.

Since 1998, surface areas show a decreasing trend when comparing lake surface areas from aerial photographs to high-water levels from field mapping. A consequence of this decreasing trend in water levels is the development of carbonate rinds on numerous glacial erratics at two lakes. Carbonate rinds may fully cover the tops of some boulders or occur as bands. These carbonate-encrusted boulders also are evident on aerial photographs after 1998.

Variations in precipitation, evapotranspiration, and fluctuations in groundwater levels affect the water levels in these small closed basins. Since 1950, records from the Tower Junction weather station show 1) an increase of 2°F, a trend which may be accompanied by increased evapotranspiration, and 2) a decrease of 5 inches in total annual precipitation. Peak flow dates at the nearby Lamar River USGS stream-flow gauging station have shifted from mid-June to late May since 1998. This hydrologic change may serve as a proxy and indicate changing groundwater levels. Subtle changes in variables affect the amount of water in these shallow lakes, making them sensitive indicators and tangible reflections of climate trends.