LARGE WOOD AS A PROXY FOR IDENTIFYING CHANGES IN CHANNEL MORPHOLOGY IN MANAGED, MULTI-DAM RIVERS

The Missouri River has a long history of dam modification which has created a series of unique morphologic and depositional features related to the interaction of both upstream and downstream dams. We have quantified the changing river form and sediment dynamics over the past 60 years using repeat cross sections and aerial photographs and examine the impact major 2011 flooding has had on these historical trends. Using this data, we see a steady shift in geomorphic features throughout the river system and have identified 5 general geomorphic zones (Dam Proximal, Dam Attenuating, River Dominated, Reservoir Dominated, and Reservoir) characteristic of dam interaction. Identification of these zones on other rivers requires extensive historical data which is often not available in many areas. Our data indicate that the distribution of Large Wood can be used as a proxy for the shift in subaqueous river conditions occurring in this system and may be useful in determining river morphologic zones elsewhere.

Large Woody Debris data was counted along 160km of the Missouri River between the Garrison Dam and Lake Oahe in North Dakota. We used 0.4m resolution GeoEye imagery of the river to quantify longitudinal depositional trends, locational environment, log orientation, and log jams in 2010 and 2012, before and after the major flooding which occurred in 2011. We also measured forested bank erosion to calculate wood recruitment into the system using remote sensing vegetation data. We found a significant change in wood distribution in the transitional zone, where sediment first becomes affected by the upstream effects of the Lake Oahe reservoir. This change in wood distribution parallels changes in sediment deposition, island morphology, erosion rates, and channel dynamics. In addition, the transition zone shifted several miles upstream during the major flooding, which reflects the observed changes in sediment distribution during flood conditions. We also identify smaller, gradual changes in distribution patterns in the free flowing section of the river which broadly reflect the change in sediment regime related to the Garrison dam. These more gradual patterns were also shifted and amplified by flooding.