Rivers may be dammed in other ways, as by lava flows, by landslides, by glacial drift, etc. In all such cases, lakes may come into existence, but they are not due primarily to the activity of the river itself. =Deltas.=[79]—Where a stream enters standing water, or a slower stream, a special form of plain, _the delta_, is sometimes built up (Figs. 169, 187, and 188). Deltas and alluvial fans have much in common, and their only notable differences are those imposed by the differences in the conditions of deposition. The current of the stream is checked, but not altogether stopped, at its immediate debouchure. If it carries abundant sediment, much of it will be promptly dropped where the decrease in velocity is first felt. Such flow as there is beyond the debouchure is not confined to a definite channel, and the deposits made are therefore spread more or less on either side of the line which represents the continuation of the stream’s course. As the depth of the water into which the stream flows increases, the current diminishes. Out to the point where the depth of the standing water is less than the depth of the current, the latter affects the bottom, and the surface of the deposits made slopes gently seaward; but where the depth of the standing water is such that the projected stream current is ineffective at the bottom, all the load rolled along the bottom is dropped, and a depositional slope is established (Fig. 188), its upper edge being below sea-level by an amount corresponding roughly to the depth of the current which brings the detritus. The outer slope is relatively steep and well-defined where the detritus is coarse, and relatively gentle and ill-defined where it is fine. Thus the stream tends to construct a sort of platform in the water just beyond its debouchure. The successive deposits on the outer abrupt slope will dip conformably with its surface (Fig. 189). The finest sediment will be carried beyond the steep slope, and conform to the topography of the bottom beyond (_c_, Fig. 189). At the beginning, the top of the delta platform is at the level of the bottom of the stream’s channel at the point of debouchure, but it is gradually aggraded as water continues to flow over it. Its landward margin is presently built up to sea-level and then above it, and as the delta grows the delta-land is extended seaward (compare Figs. 188 and 189). At the same time the channel of the stream above the original head of the delta is aggraded, for the current there is checked by the aggradation of the delta. Thus alluvial deposits continuous with the delta are extended landward. [Illustration: +Fig.+ 189.—Longitudinal section of the delta at a later stage of development.]