CHANGES IN INSTREAM WOOD OVER TIME AND THE POTENTIAL IMPACT OF LOG JAMS ON FLOODPLAIN DYNAMICS IN TWO RIVERS IN INTERIOR ALASKA

In many regions, deforestation and wood removal for navigation and river management have reduced instream wood loads in comparison to historic levels. Rivers in interior Alaska provide an opportunity to investigate instream wood dynamics in a region where wood removal for navigation and infrastructure has been minimal. Although the influences of instream wood on channel processes and in-channel habitat has been relatively well studied, changes in instream wood loads over time and the potential influence of instream wood on floodplain dynamics are relatively unknown, particularly in the boreal zone. Instream wood can influence floodplain process and form by modifying sedimentation patterns, increasing overbank flooding, and controlling channel planform and morphology. We investigate instream wood dynamics and the potential influence of instream wood on floodplain dynamics in two rivers in interior Alaska, located in the Yukon Flats National Wildlife Refuge: the Dall River and Preacher Creek. The Dall River is an incised meandering river with steep banks and a relatively slow channel migration rate. It has large channel-spanning log jams which occur in tight bends. Preacher Creek is a multithread river with more braided characteristics, a faster channel migration rate, and log accumulations that occur on in-channel bars, islands, and within secondary channels. We digitize the area and determine the spatial distribution of channel-spanning log jams and log accumulations in the 1970s/1980s and in 2011 using aerial photographs. We discuss methods and the limitations of available aerial photography for identifying instream wood in this remote location. We find that we are better able to identify channel-spanning log jams along the Dall River compared to log accumulations on bars and islands on Preacher Creek. In addition, log jams on the Dall River potentially play a role in meander cutoffs and oxbow lake formation. On Preacher Creek, log accumulations on islands and bars, although difficult to identify in historic photos, may promote sedimentation and floodplain vegetation establishment. This work contributes to knowledge of the temporal characteristics and potential impacts of instream wood in the boreal zone, in a region that has not been affected by wood removal and large-scale deforestation.