SOIL HETEROGENEITY PROVIDES EVIDENCE OF SPATIAL AND TEMPORAL VARIANCE IN CLIMATE-DRIVEN LANDSCAPE CHANGE, SOUTHEAST SAN JUAN MOUNTAINS, CO, USA

The relationship between headwater streams in steep, upland catchments their adjacent hillslopes and the subsequent contribution of sediment input to axial fluvial systems are commonly unknowns about which assumptions are made and rarely verified. Moreover, spatial, and temporal variables driving when and from where sediment is originating from valley wall and headwater hillslopes are poorly understood. However, soils can provide important insight into spatial and temporal variability in these environments, including information about the periodicity and timing of erosion, deposition, and periods of relative stability. Here, we present field and lab data from two representative tributary basins adjacent to the main axial valley of the Upper Conejos River basin, a majority tributary of the Rio Grande in Southeast San Juan Mountains.

Sawmill Gulch (SMG) and Robinson Gulch (RG), are located between 3,011-3,352 m and 3,048-3,744 m and drain 2.35 and 3.83 km², respectively. The surficial geology of both basins and their alluvial fans were mapped and described. A total of 54 soil pits were dug, described-Birkeland 1999, and sampled by horizon, ~ 350 total, on identified map units. Soil samples were processed and analyzed for particle size, organic and iron content, and pH in the lab. Multiple buried soils – as evidenced by morphology, particle size and organic content - across map units indicates periods of sediment aggradation punctuated by long periods of storage on hillslopes throughout both basins. Iron content of B horizons shows a strong correlation with soil age, providing relative and calibrated ages of buried soils. Soil data and basal radiocarbon dates from hillslope deposits and alluvial fans issuing from these basins temporally correlate to local paleoclimatic shifts – derived from pollen data extracted from bog cores. However, the response of glaciated vs unglaciated slopes varies. These data suggest soil erosion is not spatially or temporally uniform either within or between even very proximal, ~1 km, basins.