The source of a river or stream is the original point from which the river flows.
The source of a river or stream may be a lake, a marsh, a spring, glacier, or a collection of headwaters. The furthest stream is called the headstream. Headwaters are small streams that create the river or stream and may be cool waters, because of shade and barely melted ice or rain. Also they may be glacial headwaters, waters formed by the melting of the ice of glaciers. The source is the farthest point of the river stream from its estuary or its confluence with another river or stream.
Where a river is fed by more than one source, it is usual to regard the highest as its source, the others being considered tributaries. Often, however, the way that rivers and streams are named is not consistent with this convention.
Near its source a river or stream may be quite insignificant and growing as more and more water drains into it.
Most sources have rushing water that meets in one place and starts moving in a straight line down land. Rivers are usually formed by the rushing water carving into the ground and creating a large stream such as the Nile River and Charles River.
Original version adapted from the Wikipedia article "Source (river or stream)" http://en.wikipedia.org/wiki/Source_%28river_or_stream%29