DIGITAL ELEVATION MODELS IN GEOMORPHIC RECONNAISSANCE: MAPPING THE EXTENT OF PROGLACIAL LAKE SWAN, NORTH-CENTRAL MONTANA

Swan Valley is a north-south-trending half-graben lying between the Mission Range to the west and the fault-bounded Swan Range to the east. The valley’s location placed it near the margin of the icecap that covered the northern Rocky Mountains (NRM) during the Pinedale glaciation (ca. 30 ka). Locke (1995) concluded that Swan Valley was glaciated by ice flowing from the NRM icecap to the east and that, in turn, the Swan Valley glacier contributed ice to the Flathead lobe to the west. Locke (1995) also hypothesized the existence of a proglacial lake within Swan Valley, impounded against the Flathead lobe, during the early stages of deglaciation. Our goal is to use digital elevation models (DEM), coupled with published mapping of Quaternary deposits, to visualize and evaluate this hypothesis. The major product of our research will be a map showing the extent of proglacial Lake Swan.

DEM data (30-m grid) was obtained from the National Elevation Dataset and re-projected to match existing state plane data. This data set was then resampled to 1-m resolution. Neighborhood statistics were used to look for trends in the surficial fabric of the landscape. A contour file representing an elevation of 1290 m, the hypothesized sill/spillway elevation, was extracted and used to create a representation of proglacial Lake Swan at its highstand.

Our DEM-based mapping helps to identify and visualize key geomorphic features related to the proglacial-lake hypothesis. The controlling sill/spillway at the southern end of the valley is readily observed, as are the giant glacial grooves at the northern end of the valley. The important morphologic differences between the knob-and-kettle topography of the valley floor and the wave-washed glacio-lacustrine deposits (once inferred to be ice-margin deposits) of the valley margins are clearly elucidated.

In summary, it is possible to produce a DEM-based map of Lake Swan supportive of Locke’s (1995) hypothesis and consistent with previous geologic mapping. Our work serves to demonstrate the value of DEMs in reconnaissance-level mapping and the initial testing of geomorphic hypotheses.