River systems around the world diversified during the Palaeozoic era, 542 million to 251 million years ago. Before this, most rivers were shallow and wide; afterwards, some featured deep, branching channels and stable islands.

Neil Davies and Martin Gibling at Dalhousie University in Halifax, Canada, reviewed literature and visited sites of sedimentary-rock formations to determine more precisely when the change occurred. They argue for the Carboniferous period, 359 million to 299 million years ago, when the rise of trees created tough root systems able to stabilize river banks and woody debris that could block rivers and promote branching.

Nature Geosci. 10.1038/ngeo1237 (2011)