With time the gully heads advance into the upland as they take on tributaries; and so at length they in part invest it and dissect it into numerous irregularly bounded and flat-topped tables which are separated by cañons (Fig. 166). At the same time the grade of the channel is becoming flatter, and its precipitous walls are being replaced by curving slopes, as will be more fully described in the sequel. It is because of this progressive reduction of grades with increasing age that the early stages of a river’s life are much the most turbulent of its history. The water then rushes down the steep grades in rapids, and is often at times opened out in some basin to form a lake where differences of uplift have been characteristic of neighboring sections. For several reasons such basins in the course of a stream are relatively short lived (Chapter XXX), and they disappear with the earlier stages of the river history. =The meshes of the river network.=—From the continued throwing out of new tributaries by the streams, the meshes in the river network draw more closely together as the stages of its history advance. The closeness of texture which is at last developed upon the upland is in part determined by the quantity of rainfall, so that in New Jersey with heavy annual precipitation the meshes in the network are much smaller than they are, for example, upon the semiarid or arid plains of the western United States. Its design will, however, in either case more or less clearly express the plan of rock architecture which is hidden beneath the surface (Chapter XVII). =The upper and lower reaches of a river contrasted.=—From the fact that the river progressively invades new portions of the upland and lays the acquired sections under more and more thorough investment, it has near its headwaters for a long time a frontier district which may be regarded as youthful even though the sections near its mouth have reached a somewhat advanced stage. The newly acquired sections of river valley may thus possess the steep grade and precipitous walls which are characteristic of early gullies and cañons and are in contrast with the more rounded and flat-bottomed sections below. Lateral streams, from the fact that they are newer than the main or trunk stream to which they are tributary, likewise descend upon somewhat steeper grades (Fig. 167). [Illustration: FIG. 167.—Characteristic longitudinal sections of the upper portion of a river valley and its tributaries (after scaled sections by Nussbaum).]