Contrasting long-term trends in channel width and shoreline complexity

Drought and reservoir management in the Colorado River Watershed have decreased peak flows and sediment loads reducing the ability of rivers to change their channels. 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 complexity in Gray Canyon along the Green River, Utah. We quantified channel width and shoreline complexity for each year of available imagery. Between 1938 and 2021 peak flows decreased by 34% and channel width decreased by 18% confirming observations elsewhere in the system of decreasing width in response to decreasing flows. 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. Since 2006 there has been no further net increase, suggesting that room for new island formation may 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 easily be used to determine whether the increases in shoreline complexity we observed at Gray Canyon are matched elsewhere.