This term is employed to express the interruptions which hinder or prevent the normal development of a river system. The diastrophic changes and their effects we have already considered, but there are others which should be mentioned. A change of climate from moist to arid greatly interferes with the development and adjustment of a river system. Many stream channels are abandoned and others are occupied only after rains, while the reduced flow in the permanent streams diminishes their erosive powers. Large areas, like the Great Basin region, may have no outlet to the sea, because the mountain streams all lose themselves in the desert sands. Lake Bonneville (see p. 219) had an outlet until the increasing dryness of the climate so lowered its waters that the outlet could no longer be reached, evaporation exceeding influx. Great lava flows may obliterate the drainage system of a region and compel the establishment of an entirely new one, as has happened in southern Idaho and southeastern Oregon, a region of exceedingly immature topography and drainage. Extensive ice-sheets, by spreading a thick mantle of drift which fills up the valleys, may produce the same effects as lava flows, except that the drift is more easily removed.

In the northeastern United States many streams have been displaced by the sheets of glacial drift, and forced to seek new channels at a comparatively recent date; they still preserve all the signs of youth, such as deep, trench-like gorges (see Fig. 58), waterfalls, and rapids. The larger rivers have, for the most part, been able to reoccupy their old valleys, but the smaller streams have generally been compelled to excavate new channels.