The term 'river source' refers to the beginning of a river, which is usually found in the form of a HEAD or headstream. A HEAD is a four-letter word that describes the point where a river originates, while a headstream is a ten-letter word that describes a small stream that feeds into a larger river. The source of a river is typically located in higher elevations, such as mountains or hills, and is often the source of the river's water supply.