Connecting tributary erosion to downstream floodplain forest establishment in a large river network

Sediment eroded from the headwaters of a large basin strongly influences channels and ecosystems far downstream, but the connection is difficult to trace. Disturbance dependent riparian trees are thought to rely upon floods for creation of the sand bars necessary for establishment, but pulses of sediment should also promote formation of this habitat. Here we explore the hypothesis that cottonwood forest along the Green and Yampa rivers in Utah and Colorado are dominated by trees established a century ago during a period significant headwater erosion, in order to expand understanding of the role of sediment connectivity in governing ecological processes. Historical analysis suggests that three key tributaries of the Yampa underwent significant historical erosion from roughly 1880-1940. Establishment of major portions of several downstream cottonwood forests occurred concomitantly to this period of arroyo incision, and the area of forest dating to that time is much greater than can be explained by high flows alone. Viewed collectively, our findings establish demonstrable links between tributary morphological processes and distal downstream ecological processes, links we contend are best illustrated with a sediment-ecological connectivity framework. Broadly, we argue that this framework facilitates consideration of morphological and ecological processes at the watershed-scale as inextricably linked.