Falls may originate in still other ways. If for any reason a stream is forced out of its valley, it may in its flow find entrance to another valley, or to another part of its own valley, over a steep slope. If the structure of the slope favors, a fall may speedily develop. The Falls of St. Anthony are an example, the Mississippi having been turned out of its earlier course by deposits of glacial drift. Again, if an obstruction of any sort, such as a flow of lava, dams a stream, rapids or falls are developed where the water overflows the dam. When a main valley is notably deepened by glaciation the drainage from tributary valleys may fall into it, if the tributaries were not equally deepened. Falls which originated in this way are common in the western mountains of the United States, as well as in most mountain regions recently affected by local glaciers (Fig. 118). One waterfall often breeds others. Thus where a fall recedes beyond the mouth of a tributary stream, the tributary falls. The Falls of Minnehaha, on a small tributary to the Mississippi, near Minneapolis, may serve as an illustration. In such cases the falls may not develop from rapids. Once in existence, the fall of a tributary follows the same history as that of a main stream. [Illustration: +Fig.+ 117.—Kepler’s Cascade, in the Yellowstone Park. The jointed and fractured character of the igneous rocks occasions a series of falls and rapids. (Iddings, U. S. Geol. Surv.)] Streams which have falls are relatively clear.[51] If a stream favorably situated for the development of a fall carried a heavy load, deposition would take place below the rapids, and the tendency would be to aggrade the channel at that point and so to prevent the development of the fall. Falls occur only on streams which have relatively high gradients. This means that the streams which have falls are well above base-level, and streams well above base-level are young. Falls therefore are a mark of topographic youth. [Illustration: +Fig.+ 118.—The Upper Yosemite Falls.]