POST-GLACIAL ALLUVIATION IN TWO VALLEYS IN ROCKY MOUNTAIN NATIONAL PARK: THE ROLE OF GLACIATION, FLUVIAL ACTIVITY, AND BEAVERS

Historical variability of Holocene sediment aggradation was examined in two low-gradient, unconfined valleys, Beaver Meadows and Moraine Park, on the east side of Rocky Mountain National Park (RMNP). This study served to determine the volume and rates of valley fill and the role of beaver in sedimentation. In addition to examining landforms, we used geophysical techniques, including ground-penetrating radar (GPR) and seismic, aerial photo analysis, and sediment analysis from hand-augered cores and cutbanks. Moraine Park is a low-gradient (~1%) glacial trough, 3 km long and 1 km wide, flanked by lateral moraines. Beaver Meadows, with a ~3% gradient, is flanked by one of Moraine Park’s lateral moraines on the south and bedrock on the north and is ~2.5 km long and 0.1 km wide with a ~1 km long side valley to the north. Both valleys were glaciated, but only Moraine Park was covered by the Pinedale glaciation. Geophysical analysis of bedrock, glacial outwash, and alluvial fill indicate that bedrock-glacial fill contact is ~10-20 m below the surface and the alluvial fill-glacial fill contact is located ~5-10 m below the surface. Carbon-14 dating of wood and charcoal from cores and cutbanks yields sedimentation rates ranging an order of magnitude from ~0.02-0.2 cm/yr, which indicates spatial and temporal variability in sedimentation and sporadic rather than uniform sedimentation. Historical surveys indicate that beavers were prolific in both Moraine Park and the aptly named Beaver Meadows in the middle of the last century; however, currently there are no active beaver dams in either valley. Silt and clay packages, from 0.1-0.5 m thick, are prolific within the top 2 m of both valleys and are interbedded with fluvial sands and gravels. Because of the low probability of hillslope sediment inputs or log jams in these valleys, the fine sediments are interpreted as beaver pond deposits. Topographic steps in the valley profile are also consistent with filled beaver dams. These findings support the model that beaver have been an integral part of the formation of these valleys through facilitating deposition of abundant fine sediment by artificially lowering channel gradients, as well as altering channel complexity and planform.