Lake Sediments and Fire History in the Helena Valley

Abstract

Positioned on the outskirts of the Big Belt Mountains of Central Montana, Lake Helena is a man-made waterbody whose sediment can unlock secrets about the past. This region occupies the lowest areas of Helena Valley, and was formed when the Hauser dam was built across the Missouri River in the early 1900s. Prior to the flooding, the lowest areas in the basin contained wetlands and some small, lakes fed by springs. We targeted these lowest areas of the basin (as they appear in a pre-flood era 1899 map) and have obtained several cores providing a continuous sediment record spanning into the early Holocene. We obtained a 210 cm core in March of 2017, and another 210 cm sediment core in Feb 2018. Preliminary analyses show the presence of what is likely the Mazama ash (~8000) years and observable changes in sediment type, organic content and charcoal from early Holocene, through prehistoric and post settlement time to the present. Detailed analysis of charcoal from the newest core will include charcoal size, abundance and type (grass or wood), and will provide a record of fire history (frequency, intensity) for the Helena valley. This data will be compared with a fire history record obtained from a lake on the east side of the Big Belts (K. Gildner) and with climate records from archeological sites in the Big Belt mountains.