🏞️ Rivers and erosion

A river carries tiny bits of sand, gravel, and rock along with its flowing water, and those bits act almost like sandpaper, slowly scraping and grinding away at the riverbed underneath. Over an incredibly long time, that constant grinding cuts the riverbed lower and lower, carving a deep channel down into rock that once seemed impossibly solid. The same river, given millions of years, can carve a canyon so deep and wide that it becomes one of the largest visible features on the whole planet.

Water flowing even slightly faster on one side of a river than the other starts wearing away that faster bank a little more, while slower water on the opposite side drops sand and gravel instead of carrying it away. That small imbalance grows over time, pushing the river's path to bend more and more in that direction, curve by curve. Once a river starts curving, each curve tends to make the next one more likely, which is exactly why rivers so rarely flow in a perfectly straight line for very long.

Erosion is the slow wearing away and carrying off of rock and soil by water, wind, or ice, moving tiny bits from one place and dropping them somewhere else entirely. It happens so slowly that you would never notice it changing from one day to the next, yet over hundreds or thousands of years it can reshape entire landscapes, flattening mountains or carving deep valleys. Erosion never really stops, it is constantly happening somewhere on Earth, even in places that look completely unchanged from year to year.

As a river reaches flatter ground, or empties into a lake or the sea, it slows down and can no longer carry all the sand, mud, and tiny rock pieces it picked up along the way, so it drops that material behind. Over a very long time, layer after layer of dropped sediment builds up and can pile into wide, flat plains or a fan-shaped patch of new land right where the river meets open water. Some of the richest farmland in the world sits on soil built up entirely from sediment that rivers carried and dropped there over thousands of years.