An estuary is a drowned part of a river valley in a lowland area. Estuaries occur because the land sinks or the sea level rises. That may seem weird, but it has happened quite recently due to the Ice Age. Most of the rivers around northern Europe and northern North America are estuaries (although in mountainous regions drowned valleys are called fjords).

In an Ice Age, much of the water in the oceans evaporates, makes clouds and falls as snow. The snow turns t0 ice and never returns to the oceans. As a result, the sea level falls. Rivers outside the region of ice sheets, still flow, but now they find the sea level is lower, so they cut into their beds to get to the new sea level. Then, at the end of the Ice Age, the ice melts and sea levels rise, so that the valleys near the sea are now flooded – that creates a drowned river valley, or estuary.

The video below shows how a river basin works. At the end you will see an estuary which is now filling in the sand, silt and mud and so is sand and mudbanks at low tide.