GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION STANLEY A. SCHUMM AWARD: FLOODPLAIN FOREST ESTABLISHMENT AND LEGACY SEDIMENT WITHIN THE YAMPA RIVER BASIN, NORTHERN COLORADO

Cottonwood floodplain forests along the Yampa River, the last lightly regulated major river in the Colorado River Basin, are diverse ecosystems providing essential ecological and human benefits disproportionate to forest size. These forests are disturbance-driven ecosystems dependent on sediment-laden floods that form new bars for seedling establishment. Across the Colorado River Basin, cottonwood floodplain forests are declining. On the Yampa River, we hypothesize that cottonwood forest establishment is linked to a sequence of events beginning with arroyo incision in erosive semiarid low-elevation tributary watersheds in the Little Snake River Basin. No published records of widespread arroyo incision in this basin exit, and the influence of legacy sediment on cottonwood establishment in largely natural systems remains a critical knowledge gap. Nearly 60% of the present annual Yampa sediment load is derived from Sand Wash, Sand Creek, and Muddy Creek in the Little Snake River Basin. Here, we use historical maps, aerial photographic analysis, and field measurements to estimate both the timing and the volume of sediment exported from these three tributary watersheds during a period of active arroyo incision. Preliminary analysis indicates that Muddy Creek and Sand Wash incised, widened and then narrowed over the last 140 years. This suggests that sediment export from the tributary watersheds has declined substantially since the period of active arroyo incision. Early analysis of historical maps and aerial photographs indicates active arroyo incision occurred from ~1880-1940. Estimates of cottonwood forest age in Deerlodge Park, an area of particularly extensive cottonwood floodplain forest on the Yampa, indicate that most of the forest was also established during this period. Correlation between forest establishment dates and tributary arroyo incision provides early support for the hypothesized linkages between upstream erosion and downstream forest establishment.