Contrasting long-term trends in channel width and shoreline complexity

Drought and reservoir management in the Colorado River Watershed have decreased peak flows, reducing the ability of rivers to erode their banks and remove encroaching vegetation. Multiple studies have documented the resulting decrease in channel width, but less attention has been paid to long-term trends in shoreline complexity, including the number and size of islands. We used a sequence of aerial photographs and satellite images collected in 13 different years to measure decadal trends in channel width and shoreline complexity for the Gray Canyon segment of the Green River, Utah. Between 1938 and 2021 peak flows decreased by 34 % and channel width decreased by 18 %, confirming observations of decreasing width in response to decreasing flows from elsewhere in the watershed. Over the same period, however, shoreline complexity increased by 5.5 % and the number of islands almost tripled, indicating that merging of islands into the encroaching floodplain was outpaced by formation and growth of new islands. The increase in shoreline complexity occurred between 1938 and 2006. No further net increase has been observed since 2006, suggesting that room for new island formation could now be limited in the narrower channel. Sequences of channel delineations already mapped to quantify long-term changes in channel width at other sites could be used to determine whether the increases in shoreline complexity we observed at Gray Canyon also occur elsewhere.