Streams are any size body of moving surface fresh water driven towards sea level by gravity (force of attraction between two masses). Water scientists refer to all bodies of flowing surface water as streams regardless of size, yet in common language, streams are considered smaller than rivers. Stream systems are networks that collect fresh water runoff from the land and carry it to the ocean. Together, tree-shaped systems of small branch streams drain vast areas of the continents into large rivers. Stream systems of all sizes erode (wear down) sediment (particles of gravel, sand, and silt) along their courses and carve complex patterns into the landscape. They wear down slow-rising mountains and fill valleys and lowlands (low and level lands) with layers of sediment. Stream systems change character along their courses. Steep mountain streams feed shallow elevated streams that in turn flow into meandering rivers that...