The uplift of base-leveled beds, especially if the beds are tilted so as to bring layers of unequal resistance to the surface at frequent intervals, affords conditions favorable for extensive adjustment. The numerous wind-gaps in the mountain ridges, representing the abandoned courses of minor streams, and the less numerous water-gaps, which indicate the resistance of large streams to structural adjustment, are instructive witnesses of the extent to which adjustment has gone. So extensive has been the adjustment among the streams of the Appalachian Mountains that there is probably no considerable stream in the whole system which has not gained or lost through its own or its neighbors’ piracy. The history of the rivers of the Appalachian Mountains has been further complicated by a considerable amount of warping during the periods of uplift.[63] [Illustration: +Figs.+ 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]