POST GLACIAL LANDSCAPE EVOLUTION IN THE UPPER CONEJOS RIVER BASIN, SE SAN JUAN MOUNTAINS, CO, USA

Geomorphic studies of fluvial terrace and alluvial fan deposits and processes rarely incorporate direct observations of the hillslope processes that contribute to their evolution. Here, we examine the soils, sediment and geomorphology of the hillslopes of two representative upland sub-basins of the Conejos River, a tributary of the Rio Grande in the Southeast San Juan Mountains. The chosen basins, Sawmill Gulch (SMG) and Robinson Gulch (RG), are located between 9,880-11,000 ft and 10,000-12,285 ft and drain 2.35 and 3.83 km², respectively. The surficial geology and geomorphology of both basins and their alluvial fans was mapped and described. SMG is characterized by a broad, u-shaped valley with lateral and recessional moraines and small (0.03 km²), shallow sloped (average 4°), sheet flow-dominated fan deposits. RG is a v-shaped valley flanked by talus slopes with nivation hollows in the upper reaches of the basin, and exhibits a larger (0.09 km²), steeper (average 7°) alluvial fan complex at its outlet that is dominated by debris flow deposits. A total of 54 soil pits were dug and described (Birkeland 1999) on identified map units. Samples (~350) were processed for particle size (pipette method), loss on ignition, pH and Fe extraction. Overall basin hillslopes are characterized by thick, well developed soil profiles often exhibiting buried soils. Radiocarbon ages and well-developed soils in broad flat valley bottom deposits within both basins indicate sediment storage since ~7200-10600 +/- 57 years BP. A majority of mapped colluvial slopes in RG are characterized by relatively weak soil development and date to ~763-2000 +/- 36 years BP whereas the colluvial slopes in SMG are characterized by more well developed soils and date to 1881-3100 +/- 83 years BP. Alluvial fan deposits within these basins date to 2400-4000 +/- 44 years BP whereas alluvial fans at basin outlets date to 8500 -9600 +/- 40 years BP. These data suggest younger sediment that was mobilized off of hillslopes in the late Holocene has largely been trapped within these sub-basins. Together these data begin to illustrate the complexities of the post glacial landscape hillslope and valley bottom evolution in both space and time.