We Finally Know Why Some Rivers Split

What may seem like an easy question to answer — "Why do rivers split?" — has actually stumped scientists for well over a century. It's the kind of thing we take for granted, after all, it seems to be a natural feature of a natural process. The split of a single channel of water into two channels is called a bifurcation, and several famous rivers have it. The Rhine, the Mississippi, and the aptly-named Torne in Sweden all split into two. From a geological perspective, however, such features depend on specific conditions that were elusive to researchers for a long time. 

The causes of river bifurcation have been unveiled in a recent scientific study from the University of California, Santa Barbara. By analyzing nearly four decades of satellite imagery and the geological data of 84 different rivers, the team discovered a particular characteristic that causes a river to split. According to the study's lead author, Austin Chadwick, multiple river channels can develop from a single one when the erosion of one of its banks exceeds the amount of sediment deposited on the opposing, downstream banks. The result is a river that widens over time and eventually splits into two separate channels. 

While the process is easy to describe, it's a bit tricky to visualize. Logically, the powerful flow of a river should always carve out a single path, since flowing water prefers the path of least resistance. However, the conditions that Chadwick and the research team describe are rarely extreme enough to cause stable, long-lasting bifurcations, which explains why only a few major rivers with permanent bifurcations are recognizable by name. To understand how erosion and sediment deposits can cause a major river like the Rhine to split in two, we need to consider how erosion shapes and breaks a river's path.

Rivers don't always follow the same paths. Instead, rivers are dynamic phenomena, continuously eroding the land and carving new paths to follow. As rivers carve their paths, they lift up sediments and deposit them further downstream along their banks and river beds. This balance between erosion and deposition is partly the reason why rivers appear to maintain a consistent width, with the widening often only noticeable over long distances.

When a river's rates of erosion and deposition are balanced, it will remain in a single channel, or "thread." The Amazon is one such example. Over its 4,000-mile journey from the Andes to the Amazon Delta in Brazil, thousands of smaller streams and tributaries flow into the single-thread body of the Amazon. As smaller threads join into larger threads, the vast Amazon River network twists and turns in different directions, but the main body of the Amazon River generally remains a single thread. Thus, for the majority of its length, the Amazon's rate of erosion is proportionate to its rate of deposition.

When erosion does outpace deposition, a river splits into multiple threads. At first, the high erosion rate will noticeably widen the river by carrying sediments from the banks downstream. But rather than getting deposited into banks further downstream, the bulk of the sediments get deposited into the middle of the river bed. Once these deposits in the middle of the river build up, they can rise above the surface of the river and create multiple threads. Should the new threads rejoin, an island in the middle of the river is created. Should those two channels diverge without rejoining, the river splits into two rivers.

Rivers may split into multiple threads, but those splits are often fleeting. In river deltas, for example, multiple threads are constantly born and abandoned, since the river's interaction with the ocean tides and loose sandy river bed is in constant flux. Seasonal changes in water supply can also create new threads that meander away from old ones.

Human activity in particular has a major impact on river paths. Hydroelectric dams can change a river's water supply in unnatural ways, and the effect they have on multi-thread rivers provides a clear illustration of the consequences. When dams cause the water levels to drop, multiple threads may dry up, with the ultimate outcome of turning a multi-thread river into a single-thread river. This rapid transformation can change the landscape, leading to the loss of farmable land for its inhabitants. The Mississippi Delta is one stark example of the negative effects of dam construction, where significant land loss has been tied to dam construction higher up the river.

The emerging insights into multi-thread rivers may be used to aid in river restoration projects. As restoration projects seek to restore damaged ecosystems, the discovery of how an imbalance in erosion and deposition can create multi-thread rivers could completely shift the strategies employed. For instance, the University of Santa Barbara study suggests that a multi-thread system requires roughly 90% less time and space to reestablish itself compared to a single-thread system. The more we learn about rivers and why some split, the more we can manage our impacts on the natural systems that drive them.