There, the channels tend to be bounded by huge levees that have built up over time. From there, a river will spill into large meandering channels that extend out on the floor of the continental slope and the continental rise. The underwater canyons cut into the continental shelf can be compared to the headwaters of a river system. At the time, scientists compared them to mature river systems on land, such as the lower reaches of the Mississippi River. Many extend out into the ocean from the mouths of major rivers like the Amazon and the Congo, following tortuous routes across the thick sediment on the seafloor. These submarine channels were almost completely unknown until the 1980s, when sonar mapping of the seafloor began to reveal them. “If you drained all the water away, it would look exactly like a river system with bends and meanders, except there are no trees along the banks,” says Dan Parsons, a sedimentologist at the University of Hull, UK, who travels the world to study undersea rivers. Ultimately, they spill out onto the abyssal plain in the ocean depths in similar ways to a river estuary. They follow meandering paths and can even change course, resulting in abandoned sections similar to oxbow lakes. The rivers carve valleys into the sea floor. Smaller rivers called “tributaries” feed into larger ones. In many ways, undersea rivers are similar to the rivers we see on land. But these undersea rivers are among the least understood phenomena on our planet. The largest are several miles wide and run for thousands of miles out into the ocean depths, where they provide vital sustenance to the creatures living there. They have been found off the coasts of Greenland, the Amazon, the Congo and Bengal, to name a few. Similar channels can be found etched into seabeds all over the world. Beyond the mouth of the canyon, the flow has cut a valley into the sea floor that extends for nearly 200 miles before spilling onto the abyssal plain of the deep ocean. Stretching away from the coast of California, the Monterey Canyon has been shaped over millions of years by this bizarre “undersea river”. The reason? It lies two miles (3.2km) beneath the surface of the Pacific Ocean. Yet this dramatic natural wonder has never appeared in any tourist photographs, nor does it feature on any maps. It is a torrent to rival the mighty Colorado River that carved out the Grand Canyon. The river cascades through steep-sided gorges and churns around isolated towers of rock, before winding across a vast plain beyond.
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