SEDIMENT-ECOLOGICAL CONNECTIVITY: EXPLORING THE LINKS BETWEEN TRIBUTARY EROSION AND FLOODPLAIN FOREST ESTABLISHMENT IN A LARGE RIVER BASIN

Sediment loads throughout the Colorado River Basin are declining due to disruptions in sediment connectivity that are the result of both dams and land management designed to mitigate erosion in the headwaters. Cottonwood floodplain forests, ubiquitous but decreasing basin-wide, are disturbance-driven ecosystems dependent on sediment-laden floods for establishment. Here we investigate the linkages between tributary erosion and downstream cottonwood floodplain forest establishment in the Little Snake/Yampa River Basin of northwestern Colorado and southwestern Wyoming to understand the role of sediment connectivity in governing ecological processes. Analysis of historical aerial photographs and documents indicate that three key tributaries of the Yampa experienced significant historical erosion in the form of arroyo incision in the late 19^th and early 20^th century. Preliminary results from x-ray fluorescence (XRF) analyses show identifiable differences in the geochemical signature of sediment from each tributary, supporting the use of geochemistry as a sediment fingerprinting tracer to determine the provenance of mainstem floodplain sediments. Dendrochronology results indicate that the establishment of major portions of several downstream cottonwood forests on the Yampa and Green Rivers followed this period of arroyo incision, often by decades. Viewed collectively, our findings suggest that a sediment-ecological connectivity framework links tributary processes (e.g. extreme erosion) to downstream ecological processes (e.g. forest establishment). Using such a framework, we propose that sediment wave(s) of sand-sized material initiated by historical headwater erosion in the Little Snake Basin were transported >100 km downstream over multiple decades and resulted in construction of geomorphic surfaces necessary for forest replenishment along the Yampa and Green Rivers. Development and utilization of a watershed-scale sediment-ecological connectivity perspective highlights the value of sediment as a critical ecological resource to be managed jointly with flow in order to ensure the maintenance of vital riparian ecosystems.