Streams (bodies of water moving downhill in a channel) are the most effective erosional agents on the surface of the earth. Even in deserts, they are responsible for most of the landscape features that develop. 

When water hits the surface, it will either infiltrate or run off. The amount of runoff is controlled by a series of factors, such as type of materials, degree of saturation, slope, evaporation, amount and type of vegetation etc. As the water first runs off, it is not channeled but travels as sheet flow or sheet wash. In barren areas, this sheet wash will cause sheet erosion. Ultimately water collects in rills and gullies which continue to enlarge their channel. The water that is collected by these various channels runs off downstream. 

The drainage system of a river consists of a watershed, an area where the stream collects water and sediments, separated from other watersheds by a high point called a divide. The general appearance of a stream on a map is very akin to a tree and the most common drainage patterns are dendritic. In the upper portion of its watershed, a stream acts mainly as a collection system for both the water and the sediments it erodes. The main portion, or trunk of the stream, transports water and sediments. In its lower reaches, at its mouth, the stream deposits sediments. 

The ability of a stream to do geologic work is mainly a function of its kinetic energy . This, in turn, largely depends on them mass and velocity of the water and sediments. The overall velocity of the water is, in turn, controlled by a host of factors, including gradient (slope), the amount of friction in the channel, (itself controlled by the channel shape and roughness), the discharge,(amount of water), and the sediment load to mention but a few. In the upper reaches of streams, near the headwaters, the gradient is steeper, the discharge is smaller, and the channel is generally rougher. In the lower part of a stream where the reverse is true, velocity and energy rarely decrease despite a much lowered gradient, and a stream still transports the materials it eroded in its upper reaches because of increased discharge.