As small creeks flow downhill they merge to form larger streams and rivers. Rivers eventually end up flowing into the oceans. If water flows to a place that is surrounded by higher land on all sides, a lake will form. If man has built a dam to hinder a river's flow, the lake that forms is a reservoir. The water in a river doesn't all come from surface runoff.
Rain falling on the land also seeps into the earth to form ground water.
At a certain depth below the land surface, called the water table, the ground becomes saturated with water. If a river bank happens to cut into this saturated layer, as most rivers do, then water will seep out of the ground into the river. Look at the diagram below. Saturated, water-bearing materials often exist in horizontal layers beneath the land surface. Thus, some of the water in rivers is attributed to flow coming out of the banks.