POST-LAST GLACIAL MAXIMUM LANDSCAPE EVOLUTION OF THE UPPER CONEJOS RIVER BASIN, SAN JUAN MOUNTAINS, CO, USA.
Soil development combined with sediment storage and erosion in small, upland watersheds are key landscape processes for which assumptions are frequently made but rarely verified in landscape evolution studies. However, processes such as these, acting in small, steep catchments, can strongly influence the landscape evolution, ecology and hydrology of major watersheds, because they impact when and where sediment is being generated, stored and released throughout a landscape. Thus, before we can fully understand large-scale watershed dynamics, we must first describe the spatial and temporal variability of processes acting in the small watersheds that comprise most of Earth’s land surface. To this end, here I present field and lab data pertaining to the surficial geology and soils of two representative steep upland catchments, Sawmill Gulch and Robinson Gulch, of the Upper Conejos River basin, a major tributary of the Rio Grande in the southeastern San Juan Mountains. 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 the alluvial fans issuing from them onto the Conejos Valley floor, were mapped and described. Then, in order to determine the post-glacial history of each of the identified surficial geologic Map Units, a total of 54 soil pits were dug and described, paying careful attention to buried soils as indicators of past intervals of sediment aggradation and stabilization. In turn, based on overall soil development, combined with 14C dating, a soil stratigraphic framework was developed for the two watersheds, identifying key times when sediment was aggrading or eroding throughout each of the Map Units. Finally, soils were sampled and those samples (~350) were analyzed in the lab for particle size, loss on ignition (organic content), extractable iron, and pH. These laboratory data were then employed to explore temporal trends in soil development throughout the mapped watersheds. My work identified fifteen discrete map units in the studied basins, highlighting the complex suite of processes acting on these landscapes since the Last Glacial Maximum (LGM). For example, this work reveals that, contrary to traditional thought, unglaciated tributaries act as significant sources of sediment to the main valley of the Conejos River, likely due to previously unrecognized periglacial (e.g. thermokarst) processes. Examining the detailed subsurface stratigraphy of the two basins revealed that extensive wetlands (i.e. fens) have been stable and storing organics since the Early Holocene. After the mid-Holocene, erosion in both basins was largely restricted to steep slopes below 11,000 ft elevation. Overall changes in aggradation and erosion in the study basins appear to be linked to episodic Holocene climate change, whereby climate instability marked by centennial scale climate switching, led to three primary periods of aggradation (11.5-7.0; 6.5-4.0, 3.5-0.5 ka cal. yr.) whose ages match those of landforms and deposits of the Conejos River valley. Soil development-time trends throughout the basins are varied. Some soil properties, like organic content, correlate well with soil age, regardless of the map unit examined. Other properties, like soil color, exhibit no consistent trends with time, suggesting that other soil forming factors – like slope – override the influence of time. When soils with similar slope or parent material properties are considered alone, time trends become more evident in some cases. These chronofunctions suggest that landscape position (e.g. slope) and parent material (e.g. glacial sediment) seem to be the primary soil forming factors that influence the divergence of soil development trends over time in these watersheds. Overall, the results of this dissertation, have implications for how ongoing global change will influence watersheds like those of the Conejos River. I infer that global climate change – which likely will lead to increased extreme climate conditions could produce large pulses of sedimentation that could significantly alter the morphology and hydrology of the Upper Conejos River, a major contributor of water and sediment to the Rio Grande. Continued use of these lands for open grazing could further disturb these sensitive ecosystems. Ultimately, this work represents a novel combination of surficial geologic mapping and soil stratigraphy that allows new insights into the relationships between the evolution of small watersheds and the delivery of sediment and water to the Conejos River since the Last Glacial Maximum.