Arroyos are the vertical-walled, flat-floored channels of ephemeral streams of the semiarid Southwest formed both since 1880 and during some past ages. Arroyo-cutting takes place because the plant and soil mantle is poor or destroyed and permits the water of violent showers to run off too fast and form torrents in trails, ruts, ditches, and stream beds. Filling occurs when the runoff is slowed by vegetation so that it drops its load. Thus both cutting and filling are controlled mainly by the condition of the plant cover, whose natural control, in turn, was moisture. Vegetation was poor, and channeling took place during droughts, filling occurred during climatic transitions, and soil formation during relatively moist ages. The modern impairment of the vegetation coincided with a great increase in livestock and is a consequence of overgrazing and human activities. It is not a result of drought, for the primeval vegetation still remains or has recovered in areas protected from livestock. Arroyo-cutting was c...