We’ve all seen those images of underwater channels that seem to be outflows from continents, looking very much like the extension of rivers on to what is now the sea floor. The idea they represent drowned rivers become popular and I’ve read this on several occasions. On continental shelf systems that may be accurate to a certain degree but these channels often extend into very deep water – even the ocean floor. Seems like some scientists have set out to understand how they came about – are they ancient river valleys or something else. See https://www.earth.com/news/scientists-discover-that-submarine-canyons-are-not-formed-by-rivers/ … described as a global study that involved 2000 underwater canyons. They found conyons mostly below steep declines in the sea floor, from the continental margins to the deep sea. Why would they form in such a location, they wondered. Looking at continental margins around the world, the idea was also to look at how those coastal zone evolved over geological time. They also profess an interest in seeing how the oceans might store carbon, a hat tip to the global warming mantra.
A submarine canyon, or deep underwater valley, cuts into the continental edge and funnels sediment towards the deep oceans. The canyons appear to prefer deep slopes and do not need or require a river sitting above them. Tectonic and thermal processes seem to shape the sea floor and determine where the canyons form. They are major routes that carry sediments into the deep sea. In fact, the canyons may even form as a result of cutting through earlier sediment loads. The continental slope – the sea floor that drops from a shallow shelf system into deep water is shaped by sediment build up, or loss of sediment. Fast moving turbulent water currents, dense clouds of sand and mud that rush down slope, are common in and around canyons. Rivers that touch or come close to canyons, probably increease the amount of sediment run off. In other words, canyons are not derived from rivers, as such, but on occasion rivers can contribute to their turbulence. None of this of course takes into account the odd catastrophic event that in itself might increase the turbulent flow of sediment and water. Hurricanes, for instance, or tectonic processes.