The long profile of a mature stream approximates a smooth curve with ever decreasing gradient downstream. This profile curve may be called concave upward, or simply concave. Accurate surveys, mostly of streams in central or northwestern Europe or in the United States, show numerous major or minor deviations from simple smooth profile curves. Most major deviations can be correlated with obvious immaturity. Minor deviations can almost all be correlated with ascertained or probable differences in rock resistance, debris size, or other specific geologic or geomorphic circumstances. In a single region, gradients of mature streams are roughly inversely proportional to a function of discharge. One or more logarithmic curves can be drawn that will more or less closely approximate any given mature profile. General lowering of base level is found to produce a distal profile convexity. Probably such a convexity flattens and loses its initial prominence as it gradually extends upstream, unless localized on resistant rock as a distinct nickpoint.
The Monterey sea valley off California appears on a map to be the approximate seaward continuation of Salinas land valley, but its head is cut into alluvium deposited by Salinas River and adjacent streams, its irregular upper profile is nearer to an exceptionally steep straight line than to a logarithmic curve, and the compound Salinas-Monterey long profile shows an extremely pronounced break or nick on weak sediments at the shore line. Therefore it is considered improbable that the Monterey submarine canyon is the product of subaerial rejuvenation of the Salinas or any other land river. Most other sea valleys with steep gradients appear to be essentially similar to the Monterey; the Congo canyon is ambiguous and may be exceptional.