FINGERPRINTING RIVER AVULSIONS

River avulsion is the process whereby a channel relocates to a new position in the adjacent floodplain. Over long time scales, this process builds floodplains, and on short time scales it can be a significant flood hazard. However, not all avulsions behave the same; some tend to build new channels through flooding and sediment deposition, while others reoccupy abandoned channels. Predicting these avulsion styles remains elusive in part because river avulsions tend to occur infrequently. To overcome this issue, we have created a novel remote sensing analysis that can find avulsions in the modern record and fingerprint their effects on the landscape. Our analysis uses Landsat data from 1984-2017 to create cloud-free yearly composites that show channel and floodplain changes associated with avulsion. Using a Tassled Cap Transformation on all composite images, we then create a multi-year composite image using brightness, greenness, and wetness values. This allows us to effectively isolate and fingerprint those areas affected by the avulsion. Comparing our fingerprinting to high resolution imagery shows that we can identify most of the avulsion-related activity. We analyzed 34 avulsions from the Andean foreland basin and determined their position (x), channel width (W), and calculated their fingerprint. We found that at x/W < 200 avulsions reoccupy channels and at x/W > 200 avulsions deposit sediment and prograde. Avulsion channels dominated by reoccupation tend to be braided, whereas those dominated by deposition and progradation are meandering. It is yet unclear what controls this change in avulsion style, but we speculate that at positions of x/W > 200 there is ample fine sediment to fill the abandoned channels, which in turn forces floodplain deposition during avulsion. Consistent with this notion, avulsions dominated by deposition tend to occur downstream of the gravel-sand transition in the riverbed.