STRATIGRAPHY OF ABANDONED CHANNELS: INFERRING RATES AND FLOW CONDITIONS USING DEPOSITIONAL SEQUENCES OF CHANNEL FILL ALONG THE FLOODPLAIN OF THE EAST RIVER, CO

Abandoned channels are geomorphic evidence of channel migration across floodplains, and their stratigraphy can provide information crucial to reconstructing past environmental conditions. We combine remotely sensed imagery, stratigraphic profile descriptions and grain size analyses to characterize facies and determine rates of abandoned channel fill. The East River, CO is a meandering river, with a sinuosity that appears to fluctuate between 1.8 and 1.9 over the last 50 years, and maintains this dynamic equilibrium through channel migration and eventual cutoff. Channel avulsion and beaver activity result in numerous abandoned channels of varying ages and form. We investigate spatial and temporal trends in the stratigraphic sequence between 4 different abandoned channels of the East River, which have occurred over the last 50 years. Using imagery to identify the year of abandonment, we measured the depth of fine sediment within the channel and estimated the rate of deposition since the initial cutoff. Stratigraphic profile descriptions have the potential to determine former flow conditions, and long-term deposition rates across the floodplain within abandoned channels. Remotely sensed imagery and field observations suggest that abandoned channels along the East River appear to accumulate sediment, including organic-rich deposits, much more rapidly than the rest of the floodplain. Sequential images were used to examine the extent of hydraulic connectivity of the abandoned channels to the main channel over time. We observe a trend in cutoff evolution such that the downstream opening of the cutoff retains a higher amount of hydrologic connectivity than its upstream counterpart. Determining potential trends in the loss of hydrologic connectivity and sedimentary sequence of abandoned channels with relation to bend orientation may provide insight into the manor and rate at which they fill.