Waterfalls can begin forming in a river even in the absence of established external influences, reports a paper published this week in Nature. The findings suggest that waterfalls can instead develop through inherent instabilities in river action.

The traditional model for waterfall formation assumes that waterfalls form in response to outside factors.These can include a river crossing over onto a more easily eroded deposit of rock, tectonic uplift or changes in sea level, or the creation of ‘hanging’ valleys by glaciers. However, the mechanisms controlling waterfall formation are poorly understood and the origins of many waterfalls are unknown.

Joel Scheingross and colleagues used a small-scale, physical model of a river to show that waterfalls can self-form through internal feedbacks within the river, in the absence of external factors. The authors report that instabilities between a river’s bedrock erosion, flow and sediment transport increases the undulation of the riverbed. These undulations can deepen to gradually form a waterfall.

The authors propose that the key factors needed for self-forming waterfalls are a riverbed eroded by abrasion and river flow conditions that are common to streams on slopes with a gradient above one degree.These types of flows dominate mountain drainage networks, suggesting that self-forming waterfalls could be widespread in these regions. The authors note that more work will be required to understand where such waterfalls may develop and how self-formed waterfalls might be identified in nature.