Early forests tamed wild rivers

The evolution and spread of trees stabilised river banks and changed landscapes around the world forever, geologists say. Before the switch, broad, shallow, braided river channels could spread and migrate endlessly from side to side. Only when tree-like plants with deep roots took hold some 330 million years ago did river banks finally come under control, say researchers. Their assessment is published in the journal Nature Geoscience. When the change occured, rivers became single, deeper channels that meandered slowly across the floodplain, and only occasionally breaking their banks and carving out new channels. The landscape has never looked back - large meandering river channels have dominated the lowlands ever since. These stabilised, fixed-channel floodplains are well watered and can develop deep, organic soil, supporting rich, forested ecosystems. This new paradigm for landscape and ecosystem evolution comes from field work and an analysis of 330 published studies of river channels preserved in rock strata and exposed in cliff faces. Neil Davies and Martin Gibling from Dalhousie University, Halifax, Nova Scotia, Canada, describe the appearance of a distinctive pattern of river deposits in sedimentary rocks from the Carboniferous Period that persists in every subsequent geological period. In some places, the team noted that fossilised trunks and log jams had been preserved in channel sediments, proof that trees were growing along the banks. "The depth and diversity of rooting increased dramatically," write the authors. "This would have greatly boosted the stability of the entire floodplain." The Carboniferous Period saw complex and varied plant life flourish on Earth. In this period, thick coal deposits formed as plants died and were buried in the swampy plains. These deposits are the basis of our current carbon-based economy. Floodplains also provide the most fertile farmland on Earth. They allowed early human civilisation to develop, with settled populations and agriculture, and are now home to some of the largest and most densely populated cities.