Spatial Variations in Channel Morphology at Segment and Reach Scales, Middle Fork John Day River, Northeastern Oregon

Like many montane rivers in the western U. S., the Middle Fork of the John Day River is characterized by alternating narrowand wide-valley segments. Although the river has a low gradient (generally < 0.01) alluvial channel, changes in valley width exert a clear infl uence on channel morphology. A spatial hierarchical model of channel morphology was applied to the Middle Fork to understand reach-level and segment-level patterns of morphologic variation. Channel unit data for a 55-km long section of the mainstem channel and detailed profi les at segment transitions where the valley width widens or narrows sharply were used in the analysis. Wide-valley segments, where not signifi cantly modifi ed by human activities, have higher sinuosity, 80% more pool area, and pools up to 40% deeper, on average, than narrow-valley segments. Going downstream from a wide-valley segment to a narrow-valley segment, channel morphology changes relatively abruptly from pool-riffl e morphology, coarse pebble bed-material, and a gradient of less than 0.005, to plane bed morphology, cobble bed material, and a gradient of 0.005 to 0.010. Within one wide-valley segment, distinct reaches were delineated based on channel pattern and valley position. The more sinuous mid-valley reaches have several times more pool area, and pools more than 25% deeper, than the less sinuous valley-edge reaches. Segment-level differences appear to be related primarily to long term geologic controls, but these differences have been modifi ed by human impacts. Observed reach-level differences are related to both geomorphic controls and human impacts. In this montane setting, the hierarchical spatial model is a useful way of analyzing and understanding spatial patterns of channel morphology and aquatic habitat.