One useful way of studying a stream is to examine its longitudinal streams profile. Such a profile is simply a cross-sectional view of a stream from its source area (called the head or headwaters) to its mouth, the point downstream where it empties into another water body a river, lake, or ocean. By examining Figure 5.8, the most obvious feature of atypical longitudinal profile is its concave shape result of the decrease in slope that occurs from the headwaters to the mouth. In addition, local irregularities exist in the profiles of most streams the flatter sections may be associated with lakes or reservoirs, and the steeper sections are sites of rapids or waterfalls.