Unique among all the planets in our solar system, the earth has enormous quantities of liquid water at its surface. At any given instant, the oceans contain about 97.25% of all the water. A mere 2.05% is locked up in glaciers. Lakes, rivers, and the atmosphere combined hold less than .02% of the world's water supply. Ground water accounts for 0.68%. While interesting, these figures are misleading, because it is not how much or where water is stored that is critical to shaping landscapes by erosion, but its motion. 

Water is in constant cyclic motion, driven by the energy of the sun. Mostly evaporating from warm tropical oceans, moisture rises and is transported (generally poleward) by the winds. As it rises, most of it condenses to clouds, then precipitates, and returns immediately to the oceans. The remainder moves over land and falls to the ground as rain, snow, sleet, or hail. In its return trek to the ocean, water can take several routes. Some of the snow will form glaciers which move downhill and ultimately melt to mix their water with other surface or ocean water. Streams (rivers) collect the rain and return much of it to the oceans. Still other water infiltrates below the surface and seeps downhill as ground water. Most of this returning water never reaches the sea, for it evaporates from land and transpires from plants. Evapotranspiration plays an extremely important role. By recycling rain water, it nearly triples the available water supply on land. Because they move so slowly, glaciers and ground water contribute little to the yearly interchange of water when compared to streams; yet over long periods of time, they also affect landscapes.