🌊 What were bays before the sea arrived?

On this drive along the water, there's a place where a river meets the sea. A spot where river water and salty sea water mix is called an estuary. They swirl together like warm and cold water in your bathtub. But what was here before the sea came in? Let's follow one little river. A very, very long time ago, before there were any houses or cities, there was no bay at all.

There was a dry valley, a low dip between hills, like the dip between two pillows on your bed. Our little river ran down the middle. It carried away dirt and rocks, and the valley got deeper and deeper. Back then, so much water was frozen into giant ice that the sea sat much lower. The valley was dry, with no bay at all! Then the world slowly warmed.

The ice melted, and all that melted water made the sea rise. The sea crept up, higher and higher, and poured right into the valley! Count the hills you can see near the water. They might be the sides of an old valley like that one. The little river still flows into the bay today. And where its water mixes with the sea, fish, crabs and birds find a cozy home.