Water Always on the Move
A river is never still. Its water is always traveling, sometimes rushing and tumbling, sometimes drifting slowly, but always moving in one general direction. Rivers carry water across whole continents, carving valleys and shaping the land as they go. It is such a familiar sight that we rarely ask the basic question: what makes all that water move in the first place? The answer is one of the simplest and most powerful forces in nature.
Gravity Pulls It Down
The reason rivers flow is gravity. Gravity is the force that pulls everything on Earth downward, toward the center of the planet. It is the same force that makes a dropped ball fall to the ground. Water is no exception. Wherever water can move, gravity pulls it toward the lowest place it can reach. A river is simply water obeying gravity, always being drawn from higher ground toward lower ground.
From Highlands to the Sea
Most rivers begin in high places, such as mountains or hills, often fed by rain, melting snow, or springs. From these high starting points, gravity pulls the water downhill. The water follows the easiest downward path it can find, flowing along low channels in the land. As it descends, smaller streams join together into larger ones, and the river grows. The journey continues, always downhill, until the river reaches a lake or the ocean, which is about as low as the water can go.
Fast and Slow Rivers
Gravity also explains why rivers behave differently in different places. In steep mountain areas, the land drops sharply, so the water rushes quickly, tumbling over rocks in a fast, lively flow. In flatter lowland areas, the slope is gentle, so the same water moves slowly and calmly, often winding in wide bends across the landscape. Steep or gentle, fast or slow, every river is telling the same story: water being pulled steadily downhill by gravity.
Source
This article was written using information from Wikipedia.