Why don't rivers run out of water?

Base flow. I'm surprised that it hasn't been mentioned. Water doesn't always flow over the surface when it rains. Some percolates slowly down through permiable soil to the aquifer, which is how the Earth stores water. It's sort of like a water battery. read more

Alternately, geological changes can cause a river to shift, which means the water takes an alternate path to the sea, and the old riverbed goes dry. When either of those happen, you can absolutely places where there was once a river, but it's run out of water. read more

They do run out of water. I've lived in Texas for about 7 years now and up until about a month ago we've been in a drought the whole time. Most of the rivers were completely dry; we've had so much rain in the past few months that now nearly all the rivers and lakes are 3-10 feet above normal capacity. read more

At the same time water is leaving a river, more water from precipitation and melting snow and ice is joining it. read more