POST-GLACIAL LANDSCAPE RESPONSE TO CLIMATE VARIABILITY IN THE SOUTHEASTERN SAN JUAN MOUNTAINS OF COLORADO, USA

Geomorphic mapping in the upper Conejos River Valley of the San Juan Mountains has shown that three distinct periods of aggradation have occurred since the end of the Last Glacial Maximum (LGM). The first occurred during the Pleistocene-Holocene transition (~12.5 – 9.5 ka) and is interpreted as paraglacial landscape response to deglaciation after the LGM. Evidence of the second period of aggradation is limited but indicates a small pulse of sedimentation at ~ 6 ka. A third, more broadly identifiable period of sedimentation occurred in the Late Holocene (~2.2 – 1 ka). The latest two periods of aggradation are concurrent with increases in the frequency of climate change in the region attributed to increased El Niño-Southern Oscillation (ENSO) strength. This suggests that Holocene alpine and sub-alpine landscapes respond more to rapid changes in climate than to large singular climatic swings. Soil development and radiocarbon dating indicate that hillslopes were stable during the Holocene even while aggradation was occurring in valley bottoms. Thus, we can conclude that erosion does not occur equally throughout the landscape but is focused above headwater streams, along tributary channels, or on ridgetops. This is in contrast to some models which assume equal erosion in headwater basins.