One of the most interesting theories yet advanced is based on the suggestion that a really severe drought was not necessary to upset the economy of the Pueblo farmers.[10][39] Some dry farming was practiced and there was some ditch irrigation, but the greatest dependence seems to have been on flood-water farming in valley bottoms. This is a system whereby water is simply diverted and distributed through the fields when floods come down the valley. During periods when rainfall is deficient, although not sufficiently so to warrant the use of the term drought, steep channels, known as _arroyos_, are cut into flood plains; the water-table is lowered, and flood-water fields become useless. Not only may the fields themselves be dissected by the arroyo cutting, but water can no longer be diverted for flood irrigation. If, as seems probable, the great drought was only the climax of a period of increasing dryness when much farmland was lost through arroyo-cutting, it is not hard to understand why the Pueblo farmers might move on to more favored localities. Another theory advanced to explain the departure of the ancient agriculturists, and one which has enthusiastic supporters, is that they were driven from their homes by fierce nomadic tribes who were attracted by the wealth of food stored in their granaries.[73] Much of this thinking is based on what we know of nomadic raids in general, and the records of the terrible Navajo and Apache depredations from the middle of the seventeenth century until their comparatively recent subjugation by the United States Army. For years it has been the practice simply to accept the belief that fierce warlike tribes had preyed on the peaceful Pueblos for centuries. More recently, however, some searching questions have been asked, and this theory is under close scrutiny.[80] It is granted that the type of construction employed in the Great-Pueblo era indicates some need for defense, but it does not show against whom the defense was needed. Assuming that there were nomadic tribesmen, ready and anxious to carry away the patiently accumulated wealth of the Pueblos, we must ask ourselves what advantage they would have had over their victims which would have enabled them to carry out their depredations. If the nomads had been mounted, as they were in later times, they would have had the advantages of speed and mobility which are essential for surprise attacks—the only type which would be of much avail against a heavily fortified structure. Only much later, however, were horses introduced into the Southwest; and at this time the attackers would have had to travel on foot.