Development of Channel Complexity Facilitated by a Native Sedge, Carex nudata, Following Passive Restoration in the Middle Fork John Day River

Authors: Matthew Goslin*, University of Oregon, Patricia McDowell, University of Oregon
Topics: Geomorphology, Water Resources and Hydrology, Environmental Science
Keywords: Fluvial geomorphology, river restoration, vegetation, Oregon
Session Type: Paper
Day: 4/12/2018
Start / End Time: 5:20 PM / 7:00 PM
Room: Balcony K, Marriott, River Tower Elevators, 4th Floor
Presentation File: No File Uploaded

In the Middle Fork John Day River in eastern Oregon, the native riparian sedge, Carex nudata, expanded rapidly across the landscape following the removal of cattle grazing in the late 1990s. C. nudata now forms fringes along the edges of the low flow channel and grows as islands within the river. We employed multiple methods to investigate changes in channel morphology and planform relative to C. nudata: repeated topographic surveys, erosion pin arrays in cut banks and aerial imagery analysis. C. nudata appears to stabilize the edges of the low flow channel, but cut banks behind C. nudata fringes continue to erode such that the bankflow width and channel boundaries continue to move. Aerial imagery analysis indicates that current C. nudata islands are often the result of C. nudata fringes becoming “detached” from retreating banks rather than from initial establishment of plants in midchannel positions. Topographic surveys suggest scouring upstream of C. nudata islands and along the edges of C. nudata fringes. We propose a conceptual model in which alternative pathways of river development may be possible after the establishment of C. nudata depending on antecedent conditions where C. nudata becomes established. Alternative pathways include: 1) bank stabilization, channel narrowing and deepening; 2) formation of a compound channel with a side channel activated at high flows; 3) the formation of islands within the channel. The potential for multiple pathways of development may lead to complex patterns of river morphology and planform consistent with habitat complexity goals of river restoration.

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