MODELLING SHORELINE CHANGE AT WEST THUMB, YELLOWSTONE LAKE, YELLOWSTONE NATIONAL PARK, USA, AS A FUNCTION OF CALDERA DYNAMICS AND LAKE LEVEL VARIATIONS

Analysis of repeat aerial photographs from 1954, 1976, 1989, 1994, and 2002 of the northern shoreline of the West Thumb arm of Yellowstone Lake in Yellowstone National Park, USA, indicated geomorphic changes of this single area were spatially and temporally complex. The elevation of the surface of the lake on each of the aerial photographs varied from 0.436 meters (1.43 feet; 9/5/1994; estimated value) to 1.03 meters (3.39 feet; 8/4/1989; estimated value) above lake gage datum (2356 meters or 7729 feet; USGS). A rise in lake level should be represented by various degrees of recession of all shoreline segments, while a decrease in lake level should be represented by advance of all segments. During each interval between subsequent aerial photographs, though, some segments of the shoreline receded while other segments simultaneously advanced. The spatial distributions of erosion and advance on this section of shoreline indicate that lake level variations likely are not the only factor responsible for shoreline change at Yellowstone Lake.

The axis of maximum uplift of the ~640 ka Yellowstone Caldera is several kilometers to the northwest of the northern shoreline of Yellowstone Lake, and most of the lake is located within the caldera rim. Therefore, there should be a decrease in rates and amounts of uplift and subsidence from the northern shoreline southeast to the caldera rim, and this differential topographic change of the shoreline and the lake floor may be partly responsible for the complex changes observed on the lakeshore. During the period of record of the aerial photographs, the caldera alternated between inflation and deflation cycles, suggesting that the interaction between lake level, caldera inflation/deflation, sediment supply, and shoreline change is complex.

Bathymetric data from the northern portion of West Thumb was combined with the USGS 10-meter DEM of the West Thumb quadrangle to create a 3-D model of the northern West Thumb basin. The model was then used to test the response of the shoreline to both lake level variation and differential basin tilt to see how well each variable, as well as combinations of the variables, explained the shoreline change reflected in the periods between subsequent aerial photographs.