Rivers keep flowing to lower altitudes, towards the oceans.
River systems are similar to the blood vessels in your body. Tiny capillaries that carry blood keep merging together until all of the blood empties into large veins, which deliver the blood to your heart. Precipitation that falls in this area eventually flows downhill towards the river. What separates two drainage basins from each other are ridges of higher land. You might have heard of the Continental Divide, which runs along the highest ridges of the Rocky Mountains.