PROCESSES OF CHANNEL ADJUSTMENT ON FIVE STREAMS WITH CATTLE GRAZING EXCLOSURES IN THE BLUE MOUNTAINS OF EASTERN OREGON

Riparian corridor fencing is a common strategy for reducing grazing pressure on stream systems. Several studies have shown positive response of vegetation, fish communities, and channel morphology to reduced grazing, but there is limited understanding of the relative effectiveness of riparian fences on diverse types of stream systems. This study compares rates of geomorphic response of five streams, in the Blue Mountains of eastern Oregon, that differ in size and amount of bed material and suspended sediment, vegetation type, channel slope, sinuosity, age of fencing, and previous grazing history. A combination of quantitative and interpretive methods were used to focus on depositional processes, such as building of lateral point bars, construction of in-channel sedge benches, and vertical floodplain accretion, with the notion that the types of processes that are dominant on a given stream play major roles in determining rates of channel adjustment after grazing is eliminated. Little overall change over five to eight years was revealed by repeat surveys of channel cross-sections at two sites that are incised, and have fine bed material and large amounts of fine sediment. Some lateral migration but little overall change in cross-sectional size and geometry was shown at sites that are less incised and have coarser bed material and less fine sediment. This is probably in part because of the limited spatial accuracy of repeat cross-section surveys compared to depositional rates over short time periods. A second methodology involves detailed field descriptions of depositional surfaces, along with interpretation of depositional processes based on previous studies. This method allows estimation of relative rates of fairly slow processes for sites with little or no early baseline data. Preliminary results suggest that sinuous streams with actively mobile bed material are most likely to build new depositional surfaces, but availability of fine sediment is also necessary in order to build banks and floodplains that will support dense vegetation and be resistant to future erosion.