New research led by the U.K. National Oceanography Centre (NOC) has discovered how fast-moving waterfalls under the sea control the shape and behavior of submarine channels.

These underwater channels are the offshore equivalents of rivers, but can be much larger. Submarine channels can extend for tens to thousands of kilometers offshore, providing an important conduit for the transfer of sediment, nutrients and pollutants, such as microplastics, to the deep sea. Avalanches of sediment that flow down these channels also pose a hazard to networks of seafloor cables that underpin global communications, including the internet.

An international research team, including NOC, Universities of Durham, Southampton and Hull (U.K.), Geological Survey of Canada, and University of New Hampshire (U.S.), performed the most detailed repeated mapping of any submarine channel to date. Time-lapse surveys acquired over nine years in Bute Inlet, British Columbia, revealed a dramatic series of up to 30-m-tall, steep cliffs (called “knickpoints”) that resemble waterfalls in rivers. 

Similar features migrate at a rate of less than 1 m per year in rivers. The team observed much faster migration rates in the submarine channel: up to 450 m per year.  

Almost three quarters of the erosion in the channel resulted from these fast-moving knickpoints, leading the study’s authors to conclude that knickpoints are far more important for the evolution of submarine channels than other previously suggested controls, such as the growth of meander bends.

An abundance of repeated aerial photographs and satellite data has revealed how rivers change over time; however the challenges of surveying deep underwater, has meant that such a wealth of time-lapse surveying does not exist for submarine channels. Instead, most of our understanding has been based on scaled-down laboratory models or from one-off seafloor surveys that only capture a snapshot in time.

The powerful sediment avalanches that transit these submarine canyons also pose a threat to critical seafloor infrastructure, such as the network of telecommunication cables that carry over 99 percent of all digital data traffic, including the internet.  In 1929 an earthquake off the east coast of Canada triggered submarine flows, which severed 12 transatlantic cables. Submarine landslides and sediment flows are also a common coastal hazard in Canada’s inlets. The deposits of such flows are often used for dating major earthquake and land flood events, and therefore understanding how the channels transport these flows is a very important piece of information.