The making of an alluvial plain usually involves both erosion and deposition. When a stream has cut its channel to grade, downward erosion ceases, or more exactly, downward cutting is, on the average, counterbalanced by deposition. So long as a stream is cutting downward rapidly, it carries away whatever débris descends the side slopes. When it approaches grade, the débris which descends the side slopes tends to accumulate at their bases, and the V-shaped cross-section of the valley becomes U-shaped (see Fig. 172). At about the same time the stream begins to meander, for, having lost something of its former velocity, it is more easily turned from side to side. As it begins to meander, it widens the bottom of its valley. This is the initial stage in the development of the valley flat (2 and 3, Fig. 172). In its meandering the stream encroaches on the talus accumulations at the bases of its valley’s slopes. The side-cutting may remove all the loose débris and even undercut the bluff as at _a_, Fig. 173. The stream’s meanders shift their positions from time to time so that the valley flat is successively widened at different points. By lateral planation, therefore, a stream tends to develop a flat as soon as it reaches grade. This is the initial part of erosion in the making of a river flat, but a flat developed by erosion alone is not an alluvial plain. So soon as the flat developed by a stream exceeds the width of its channel, the water (except in times of flood) does not cover it all at the same time. On any part which it temporarily abandons, some débris (alluvium) is likely to be left. This deposit of alluvium constitutes the valley flat an alluvial plain (Fig. 174). It will be seen that the valley flat is commonly an alluvial plain from the beginning. [Illustration: +Fig.+ 172.—Diagram illustrating the transformation of a V-shaped valley into a U-shaped valley.] [Illustration: +Fig.+ 173.—Diagram to illustrate the widening of a valley flat by erosion. Compare 3, Fig. 172.] Once the valley flat and alluvial plain are begun, their further development is easily followed. The stream in flood overflows the banks of its channel. The velocity of the overflowing water is reduced, and if it has much load a part of it will be dropped and the plain aggraded. Meantime meandering and lateral planation continue. Thus the flood-plain is widened by erosion, and aggraded by alluviation, the two processes going on simultaneously. [Illustration: +Fig.+ 174.—An alluvial plain. The diagram suggests the relative importance of lateral planation and alluviation in the development of the flat.]