UNDERSTANDING SEDIMENT ACCUMULATION AND DISTRIBUTION IN JACKSON LAKE, WYOMING USING CHIRP SEISMIC SURVEYING

Measuring sediment volumes in lake basins using seismic data provides a means of estimating mass efflux as a response to tectonic and climatic forcing. Jackson Lake (WY) is amenable to studying variability in stratal development and the pace of sedimentation in a zone of crustal extension that has been modified by glacial activity over time scales that range from 10² to 10⁴ yrs. Ancient sedimentation in Jackson Lake is likely constrained by the Pinedale glaciation (~30-13 ka). Jackson Dam was built on the lake in 1911 and in addition to increasing accommodation, provides a datum for sedimentation over the last ~100 years. We collected a grid of high-resolution CHIRP seismic profiles in Jackson Lake and used these data to identify key reflectors. Using the extant lake floor and acoustic basement reflectors, we generated a minimum isopach map of the post-Pinedale sediment package within the lake. In the deepest portions of the lake, total sediment thickness above acoustic basement exceeds 120 m, which would require a minimum sedimentation rate of 0.9 cm/yr since ~13 ka. To investigate sedimentation rates for Jackson Lake since dam emplacement, we identified a candidate reflector that may represent the transition experienced by the depositional system associated with the change in outflow. A literature review of 90 reservoirs suggests a mean sedimentation rate of ~2.1 cm/yr for these engineered lakes. Most reservoirs accumulate sediment at rates <1 cm/yr, and a few exceed 10 cm/yr. In the portion of Jackson Lake nearest to the dam, a possible “Dambrian” reflector lies ~13 m below the lake floor, suggesting a potential maximum sediment accumulation rate of 13 cm/year since the early 20^th century. The spatial variability of sediment thickness suggests complex controls on sediment influx and accumulation; future acquisition of sediment cores and seismic data with greater depth-of-investigation will allow for more refined estimate of these rates.