Managing meandering waterways in a changing world

To investigate these dynamics, Greenberg and Ganti collated existing measurements of river migration rates and added data from approximately 60 additional rivers. Altogether, they compiled data on 139 meandering rivers across the globe, spanning different regions, climates, sizes and vegetation regimes. The researchers modeled each river channel as a series of line segments using satellite imagery. They could then track how these segments shifted over time to measure the river’s migration.

The leading paradigm was that vegetation slows down this migration by stabilizing the outer bank against erosion. This contrasted with experimental evidence suggesting that sediment load could be an influential factor. Bank pull is stronger in unvegetated rivers, but as Greenberg and Ganti discovered, these tend to have higher sediment supply as well, making it difficult to distinguish the relative contributions of the two processes.

But Greenberg and Ganti’s analysis revealed a clear trend: Migration was faster for rivers that carried a lot of sediment relative to their size. The model also showed vegetation slowing down river migration, as suggested by previous studies. However, the effect was much more modest, with unvegetated rivers migrating four times faster than similar-sized counterparts, rather than the 10-fold increase reported by some of their colleagues. This suggests that bar push has a stronger influence on meandering rivers than bank pull.

That said, river behavior flows from the confluence of the two processes. “You can’t have one dominate the other in a meandering river,” Ganti said. “If you don’t have enough sediment supply, bank pull will outpace bar push, and you’ll end up with a braided river. And so it’s really the balance between the bar push and the bank pull that creates these stable meandering rivers.”