Want to learn more about how meandering streams and rivers evolve? Try building one yourself and see how it behaves. That's what a team of researchers did at the University of California, Berkeley. The resulting data should help environmental scientists design more sustainable restorations for degraded river systems. And the findings could even help planetary scientists understand how stream patterns once formed on Mars and Saturn's giant moon Titan.
Lazy, meandering streams distribute nutrients to sustain a host of plant and animal species. Meandering streams also change shape over time. Wherever a stream channel takes a bend, water flows faster on the outer edge of the channel and erodes the banks. Meanwhile, the current is weaker on the inner edge, and the river deposits the sediment it's carrying, which creates sandbars. This erosion and deposition eventually creates a new path for the river.
Over the past century or so, many meandering streams and rivers have been altered, as industrialization and urbanization have spread across the landscape. Runoff of eroded soil from building construction and farming has plugged up channels, for example. Public agencies and private organizations alike have attempted to restore many of those streams and rivers by re-excavating channels to recreate meandering patterns. Most efforts have failed, however, because the streams have tended to straighten out or break up into multiple channels after just one or two floods. Environmental scientists just didn't know enough about the conditions under which a damaged river could be returned to its meandering ways.
So graduate student Christian Braudrick, geomorphologist William Dietrich, and colleagues decided to find out exactly how meandering streams evolve over time. They built a gently sloping sandbox 6 meters by 17 meters in their laboratory. After carving a channel with one bend into the sand, Braudrick and Dietrich planted alfalfa sprouts to simulate vegetation growth along the banks. Then they let water and sediment flow through the channel at varying rates over a total of 136 hours over a year-long experiment. During that time--which simulated up to 7 years of a full-scale stream flow--the stream gradually shifted its course and continued to create new bends.
Please download the latest version of the free Flash plug-in.
The team found that the two most important factors maintaining the meandering were bank strength and extrafine sediment, they report in the current issue of the Proceedings of the National Academy of Sciences. Strong banks resist excessive erosion so that the sandbars on the inner bank can keep pace with the retreat of the outer bank, Braudrick explains. "Our experiments show that bank strength is a first-order control and should not just be an afterthought." Meanwhile, he adds, the fine sediment carried by the stream builds up the sandbars, preventing the stream from cutting new channels.
The team also found that the vegetation along the stream's banks appears to control erosion. But that raises a question about other planets: What explains the meandering streambeds on Mars, where presumably no vegetation grew? It's possible, Braudrick says, that frozen soil strengthens the banks, as happens in Arctic landscapes on Earth.
"The study is a milestone," says geomorphologist David Montgomery of the University of Washington, Seattle. It advances the understanding of fundamental controls on river morphology and behavior, he says, something that "has long proved elusive in the lab." The model confirms a long-held suspicion that strong, cohesive banks are essential to the health of meandering streams, he says. "They've also kicked open the door for further experiments sure to enlighten river restoration and management."