Changes in species composition, planform and channel morphology following 20+ years of passive restoration in the Middle Fork John Day River, Oregon: the case of a native sedge, Carex nudata.

Changes in species composition and channel geomorphology following passive restoration in the Middle Fork John Day River, Oregon: the case of a native sedge, Carex nudata.

The Middle Fork of the John Day River (MFJDR), Oregon, has been the focus of both active and passive restoration efforts since the late 1990s following the establishment of private conservation areas and reforms in U.S. Forest Service cattle grazing management. In the mid-1990s, a multidisciplinary, collaborative effort between University of Oregon (UO) and Oregon State University (OSU) gathered data on geomorphology, vegetation, macroinvertebrates and fish throughout the MFJD prior to these changes in management. In 2018-2019, a renewed collaborative effort between UO, OSU and Tribes resurveyed these data in order to assess long term changes relative to differing restoration strategies (including no restoration). These remeasurements were paired with analysis of historic air photo sequences. Here we present results focused on changes in greenline vegetation and channel planform. In sites with passive restoration, greenline surveys show a shift in species composition in towards perennial, deep-rooted sedge communities. In particular, the native riparian sedge, Carex nudata, became a dominant species whereas it had previously been suppressed by cattle grazing. Aerial imagery analysis showed narrower greenline-to-greenline channel widths and increases in channel complexity metrics. These results will be discussed in the context of complimentary research focused on C. nudata specifically in which we have employed multiple methods to investigate the changes in channel morphology in association with C. nudata: aerial imagery analysis, repeated topographic surveys, and erosion pins in cut banks with C. nudata fringes. C. nudata appears to stabilize patches where it becomes established but banks continue to retreat, channel boundaries continue to move leading to compound or multi-threaded channels around C. nudata islands. We propose a conceptual model in which alternative pathways of river development become possible after the establishment of C. nudata, and the potential for multiple pathways may lead to complex patterns of river morphology and planform in the MFJDR consistent with key restoration goals.