River restoration is currently the focus of enormous investment throughout the West Coast, motivated by the imperative to enhance habitat for threatened Pacific salmon populations. This research will contribute to these efforts: Carex nudata, a native sedge that occurs throughout the West Coast, is increasingly perceived as a facilitator of restoration goals, but land and river managers lack a clear understanding of how it may alter channel morphology and what environmental variables are critical to its viability. The proposed research explores the interactions between C. nudata, a riparian sedge, and its river environment. The research is distinguished by its aim to examine both sides of this relationship: how the river's physical environment and flow patterns drive C. nudata distribution, and how C. nudata then alters the physical processes of the river. C. nudata caught the attention of investigators in the Middle Fork John Day River where it has exploded in abundance following the withdrawal of cattle grazing from river banks. C. nudata grows along the edges of the low flow channel, on gravel bars, at the base of cut banks and as islands, its dense root system allowing it to withstand strong flows. C. nudata appears to be altering channel morphology in ways that enhance river complexity, thus enhancing fish habitat and facilitating river restoration goals.
The investigators hypothesize that C. nudata facilitates the evolution of compound channels in which C. nudata stabilizes a narrow low flow channel but, unlike many plants, does not slow erosion of the cut banks, such that the bankfull channel continues to move or widen. In some cases, this process is hypothesized to lead to C. nudata patches at the base of banks "detaching" and becoming islands, giving rise to multi-threaded channels. The investigators will use multiple methods to examine these hypotheses. Changes in bank retreat and channel bed erosion/aggradation will be examined using repeated topographic surveys and erosion pins. Bank retreat and island formation will also be examined with aerial imagery. Flow velocity patterns that may explain patterns of erosion/aggradation will be examined with an Acoustic Doppler Current Profiler. With regards to C. nudata distribution, the investigators hypothesize that C. nudata patterns are driven by hydrological variables and sediment type. These hypotheses will be examined with headwaters-to-mouth surveys for C. nudata in three distinct river basins. Until the latter part of the 20th century, river geomorphology focused solely on physical dimensions of river processes. Only recently has vegetation been recognized as having significant effects on river shape and form (Hughes 1997). Nevertheless, current studies of plant-river interactions typically focus on one dimension of this relationship: i.e. either the effects of the river environment on plant communities or the effects of plants on certain physical processes. The proposed research follows Corenblit et al. (2007) who emphasize the reciprocal nature of plant-river relationships and propose the concept of biogeomorphological succession to describe the linked development of river shape and form with riparian plant communities.
