Some Edaphic and Ecological Effects of Water Spreading on Range Lands

Abstract

Striking physical features of the southwestern United States are the sheerbanked arroyos which dissect many of the broad alluvium-filled valleys. In many instances these gullies are forty to fifty feet deep; the width ranges from a few yards to several hundred yards; the length may be a hundred miles and more. The consensus is that these extensive arroyo systems have developed since 1880, and that before that time channels were discontinuous and shallow, and the water of the intermittent flash flows spread out over the grass-covered flats which constituted the valley floors (Bryan, '25, '28, '29; Cooperrider and Hendricks, '37; Thornthwaite, et al., '42). The present arroyos not only rob the valley floors of the flood water they previously received; but, by means of tributary gullies, they drain off a part of the scanty precipitation. This diminution of the water supply has resulted in a decrease in the vegetative cover and a change in the floral composition (Bryan, '28). It is the purpose of water spreading to conserve for plant production the water otherwise lost. The Indian tribes of the Southwest have spread the water of the flash flows of ephemeral streams for crop production since ancient times (Bryan, '29, '41; Stewart, '40), and the practice was adopted and used by the Spanish and other white settlers until channel entrenchment made it impractical in many instances (Bryan, '29). Water spreading on range lands is being successfully practiced in the western United States (Bennett, '39; Bingham and Monson, '37; Monson and Quesenberry, '40; Semple and Allred, '37); but, as has been pointed out previously (Gardner and Hubbell, '42), the writers know of no detailed studies on the edaphic or ecologic responses. Studies of water spreading on range lands were started in 1935 on the Navajo Experiment Station area, which lies in northwestern New Mexico in the Chuska Mountain and the Chuska Valley physiographic subprovinces of the Colorado Plateau (Reiche, '41). The vegetation and topography are characteristic of much of the country in that part of the state and in northeastern Arizona and southeastern Utah. The geology of the region as a whole has been treated by Gregory ('17); that of the Station area in particular, by Reiche (l.c.). The western part of the Station area lies in the edge of the Chuska Mountains; the eastern part is a rolling plain with broad alluvial valleys. The material which fills the valleys is outwash from the sandstones and shales of the Mesaverde formation of the mountainous uplands (Reiche, l.c.). The resulting soils are of the Crown series and vary from sand to clay. The margins of the arroyos are usually higher than the land lying farther from the channel. These natural levees lessen the difficulties of spreading water by keeping it from running back into the arroyo. The streams of the area are all ephemeral, carrying water only when there has been sufficient rain or melting snow in the drainage basin to cause runoff. The sediment load carried by this water is usually high, varying from 1 per cent or less by weight in the case of runoff from melting snow to 55 per cent or more in that resulting from heavy rains in badland areas (fig. 1). Melting snow during the period of treatment has contributed a very small pro-