A native sedge, Carex nudata, as facilitator of restoration goals: effects on channel morphology and planform in the Middle Fork John Day River

Abstract:

In the Middle Fork of the John Day River, the native riparian sedge, Carex nudata, has exploded 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. C. nudata appears to be altering channel morphology and planform in ways that may facilitate key restoration goals. I have employed multiple methods to investigate changes in channel morphology and planform in association with C. nudata: aerial imagery analysis, repeated topographic surveys, and erosion pins in cut banks with C. nudata fringes. Preliminary results suggest that C. nudata stabilizes 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 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 multiple alternative pathways of river development may be possible after the establishment of C. nudata, depending on the antecedent conditions where it becomes established (e.g. bank composition, river curvature). Alternative pathways include: 1) bank stabilization, channel narrowing and deepening; 2) formation of a compound channel with a side channel that is activated at high flows; 3) the formation of islands within the channel. The potential for multiple pathways of development after C. nudata establishment may lead to complex patterns of river morphology and planform consistent with habitat complexity goals of river restoration.