The stream system, like almost all environmental systems, is open to both inputs and outputs of various types of materials. Water enters the stream system by direct precipitation in the channel, from runoff , throughflow , and by groundwater flow . The movement of water into a stream also carries with it dissolved and solid materials eroded from the surrounding landscape, stream banks, and the stream bed. Sediments carried by streams to lower elevations are occasionally deposited and stored at numerous locations for various periods of time within the stream system before they reach their final resting place. Losses of material carried by the stream system occurs through a number of processes.
Water is lost by evaporation , seepage , and flooding .
Streamflow ends when the water carried by stream enters a receiving basin like a lake or an ocean . Sediment is lost by various types of deposition . At their headwaters , the grade of a stream is usually steep. As streams get closer to sea-level, the angle of the grade becomes more gently sloping. Near the mouth of the stream, the grade becomes almost flat.