I explored the effects of the riparian sedge, Carex nudata, on geomorphic processes in the Middle Fork John Day River, Oregon (MFJDR) as well as the environmental drivers of C. nudata’s distribution, building an integrative conceptual model of stream evolution in rivers with C. nudata.I investigated the environmental drivers of C. nudata distribution and tested the hypothesis that distribution is driven by stream power by conducting field sampling across 31 sites in the John Day and Santiam basins of Oregon. C. nudata abundance was inversely related with canopy cover and displayed a positive threshold response relative to stream power, mostly absent in streams with low stream power.
Within the MFJDR, I used repeated topographic surveys and historic aerial imagery to investigate changes in channel morphology associated with C. nudata. Repeated surveys showed continuing bank erosion and small-scale changes such as scour in front of C. nudata fringes. Historic aerial imagery revealed that C. nudata islands most often originate from C. nudata fringes becoming detached from retreating banks rather than from initial establishment in midchannel positions.
The continuing erosion of banks with C. nudata fringes raised the question of whether banks with C. nudata fringes are eroding at rates similar to banks without C. nudata. I addressed this question by establishing erosion pin arrays at 7 sites with C. nudata fringes and 7 without, measuring pins seasonally for 2 years. Erosion rates did not differ between sites with and without C. nudata. Furthermore, winter erosion was equal or greater than erosion during the spring snowmelt-driven peak flows pointing to the importance of winter processes such as freeze-thaw soil weakening that are likely independent of C. nudata patterns.
I propose a conceptual model in which alternative pathways of channel development are possible after the establishment of C. nudata: 1) bank stabilization; 2) formation of a compound channel as banks retreat; 3) the formation of islands within the channel as banks retreat and scour occurs behind a C. nudata fringe. The potential for alternative pathways can lead to a diversity of channel forms, facilitating complexity, a key goal of river restoration
