Sediment Cores in Lake Helena Show Flood Deposits and Their Mineral Impact on the Lake
The Helena Valley has experienced major floods in 1938, 1975, 1981, and 2011. Satellite imagery of the Tenmile/Prickly Pear Creek delta, at the west end of Lake Helena, shows a distinct area of fresh sediment deposited after the 2011 flood. The sediments transported in just this one event advanced the delta front more than 100 ft into Lake Helena. Observation of this large volume of flood transported sediment in 2011 led to the hypothesis that historic flood events likely caused similar sediment mobilization and deposition into the lake. We expect that sediment cores obtained from the bottom of Lake Helena will provide a record of the 2011 and earlier flood events, evidenced as distinct and traceable sediment layers delivered across the lake bottom. Two cores, one obtained near the delta and another further to the east, may allow us to quantify the extent and volume of flood transported material across the lake bottom. Historical mining in both Tenmile and Prickly Pear Creek watersheds has led to arsenic and other metal contamination in the creeks and runoff into Lake Helena. Analyzing the cores for grainsize, magnetic susceptibility, and organic content will allow us to determine if major flood events transport significant amounts of sediment to the lake and how much. Additionally, analysis for metals may tell us if these flood pulses cause increased contaminant transport into Lake Helena from source areas considered ‘stabilized’ during normal stream flow.