Every time a river floods, it subtly or drastically changes its path. Scientists have long strived to capture this complex process in mathematical models because it includes many variables, including the softness or hardness of the soil the river erodes, groundwater, and the vegetation within and along its shores.
Now a new computer simulation of how rivers evolve over time comes one step closer to capturing that complexity. The finding could help scientists better predict how floods will remodel a river’s banks and affect the people in its path.
A realistic simulation of river evolution must be able to reproduce a wide variety of meanders: the horizontal and vertical detours it makes as it travels downstream. These meanders are formed by the removal and deposition of silt, sand, mud, and rocks along the outside and inside of bends, which build up characteristic features such as scroll bars—ridges of sand and mud that accumulate on a river’s inside curve—and oxbow lakes, crescent-shaped lakes that form when a river’s main stream cuts off one of its meanders.
In their mathematical model of how a meandering river and floodplain interact over time, Bogoni et al. attempt to simulate how scroll bars and oxbow lakes form and affect meanders by tweaking variables such as the softness of the soil. At first the model river flowed in a straight line, but as the scientists made the floodplain easier or harder to erode, it wobbled, producing meandering undulations.
The most realistic meanders formed when the scroll bars and oxbow lakes left behind in previous floods were harder to erode than the surrounding floodplain, the team reports. The finding illustrates how a river’s history affects its present and future course: The traces a river leaves behind define the route it can take. (Water Resources Research, https://doi.org/10.1002/2017WR020726, 2017)
—Emily Underwood, Freelance Writer