Fig. 161, 162.—Diagrams to illustrate the effect of crustal warping on stream erosion. The dotted lines represent the profiles of the streams before deformation; the full lines, after. Erosion will be stimulated between a and b in each case, and between c and d in Fig. 162. Below b, Fig. 161, the stream will be drowned, and erosion therefore stopped. Erosion will also be stopped or retarded above a, between b and c, and below d in Fig. 162.
Sinking.—The land on which a river system is developed may be depressed relative to sea-level. In this case the sea would occupy the lower ends of valleys, converting them into bays and estuaries. A stream in this condition is said to be drowned. Of drowned rivers there are many examples along the Atlantic coast. Thus the St. Lawrence River is drowned up to Montreal, and the Hudson up to Albany. If the drowned portion of the latter valley were not so narrow, it would be a bay. Delaware and Chesapeake Bays, as well as many smaller ones, both north and south, are likewise the drowned ends of river valleys (see figures, [Chapter VI]). If all parts of a drainage basin sank equally, the velocities of the streams above the limit of drowning would not be changed, for the gradients would remain the same as before. The fact that a river’s channel is below sea-level is not to be taken as proof that the valley is drowned. Thus the bottom of the channel of the Mississippi is as much as 100 feet below the level of the Gulf, some 20 miles above New Orleans.[64]
Differential movement. Warping.—Where a land surface on which a river system is established suffers warping, some parts going up and others down, the opposite movements being either absolute or relative, various phenomena would result. This may be illustrated by the accompanying diagrams (Figs. [161] and [162]), where the profiles of the streams are represented as warped from the positions represented by the dotted lines, to the positions shown by the full lines. The velocity will be accelerated below the points of differential elevation (between a and b, [Fig. 161], and between a and b, and c and d, [Fig. 162]), but checked above (above a, and between b and c, [Fig. 162]). Above an elevation which notably checks its flow, a stream is ponded. If the ponding is slight, a marsh may develop above the obstruction; if more considerable, a lake is formed. Lakes of this class are likely to be short-lived, since the ponded waters are likely to soon overflow and lower their outlet so as to drain the lake. The elevation which ponds the stream may be great enough and rapid enough so that the resulting lake finds an outlet by some course other than that originally followed by the stream. Where a stream holds its course across an uplift athwart its valley, either with or without ponding, it becomes an antecedent stream (see [p. 169]), since it has a course assumed before the latest deformation of the crust and in apparent disregard of present surface configuration. Thus the Columbia River holds its antecedent course across areas which have been uplifted (differentially) hundreds and even thousands of feet.[65] Some of the striking scenic features of this noble valley are the result of these changes in the country through which it flows. A lesser stream would have been diverted, as many of its tributaries have been. Even its course across the Cascade ranges is believed to be antecedent.[66]
Fig. 163, 164.—Piracy stimulated by warping. Uplift along axis 1–2.
Another peculiarity of valleys and streams resulting from changes of level is illustrated in [Fig. 2, Pl. XIV] (southern California). The main valleys of this part of the coast were developed when the land stood considerably higher than now. Later the subsidence of the coast converted the lower ends of the valleys into bays or fiords. The bays were then transformed into lagoons by deposition. Subsequent rise of the land or depression of the sea allowed the drainage from the old lagoons to cut across the deposits which had converted the bays into lagoons. The result is an old, wide valley above, suggested by a young one below.
If the warpings were considerable, much more decisive changes in drainage would result. Suppose the drainage of a given region to be represented by the streams in [Fig. 163]. If there is uplift along the axis 1–2, that part of ac above the axis of uplift would be ponded, or at least have its velocity checked, while the flow of some of the tributaries of d would be accelerated, and might work back and capture the other stream ([Fig. 164]).
Crustal warping was one of the conditions under which the Tennessee achieved its present anomalous course, and its history[67] is illustrative of the complex changes which drainage suffers when warping affects the area where the rock structures are of unequal resistance. At the close of the Cretaceous cycle of erosion, when the Appalachian Mountains had been reduced to a peneplain, the waters falling in the area now drained by the upper course of the Tennessee flowed south-south-west to the Gulf in a stream (the Appalachian River, a, [Fig. 165]) the lower part of which had the general position of the Coosa and the Alabama.
To the west of the Appalachian River, shorter streams flowed west and southwest into the Mississippi embayment ([Fig. 165]) by courses which are not now definitely known. The succeeding cycle of erosion was inaugurated by uplift and deformation of the peneplain. The axis of greatest elevation (AB, [Fig. 166]) was nearly parallel to the Appalachian River, and the effect of the differential uplift was to impose a greater task on this river (a, [Fig. 166]), which flowed along the axis of uplift, than upon the rivers which flowed westward and southwestward to the Mississippi embayment. The result was that the strongest of the southwesterly flowing streams worked its head back into the drainage basin of the Appalachian River, and captured, one by one, the head-waters of its westerly tributaries, establishing some such drainage relations as are shown in [Fig. 166]. Still later, after the land area of the region had been considerably extended by the withdrawal of the sea, the Appalachian River itself was reached by the invading stream, and its waters carried away to the Mississippi Bay by a course the lower part of which is thought to have corresponded approximately with the course of the present Black River (b, [Fig. 167]).