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4 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS storm center is less than 1.5 inches. Another rule of thumb is that for arroyos to flow, the curtain of rain as viewed from a distance must be so dense that one cannot see through it. During three field seasons we were constantly on the watch for thunderstorms and when one appeared close enough to be reached in less than half an hour, we hur- ried to get to it. By seriously chasing individual storms, and even though helped by a rather intimate knowledge of local roads and geography, we were able in this period to obtain hydraulic measurements less than a dozen times. Most of them were for locations near Santa Fe, with the others at scattered points in Colorado, Wyoming, and Nebraska. As will be de- scribed later, the hydraulic characteristics measured during storms include channel width, depth, velocity, and suspended-sediment load. It was our plan to combine the measurements of hydraulic factors with data obtained in the field when the channels were dry, and with information taken from maps and aerial photographs, Slope was measured with a telescopic alidade, and channel width was deter- mined by taping or pacing. At the locations where flo.w .was measured, material comprising the channel bottoms was sampled, Drainage areas and stream orders were determined from tlle excellent planimetric maps compiled by the Soil Conservation Service from aerial photographs and available for parts of central New IvIexico. MEASUREMENTS OF HYDRAULIC VARIABLES 1:-1 EPHEMERAL STREAMS GENERAL FEATURES OF FLOW The ephemeral nature of flow in arroyos is their most impressive characteristic, Flash floods are the rule. A typical dry arroyo reaches peak discharge in less than 10 minutes, the high flood flow seldom lasts more than 10 minutes, and flow decreases to an insignificant amount in less than 2 hours. A typical flood rise of small magnitude is shown by the series of photographs in figure 4. This series was taken by Herbert 'V. Yeo, whose diligence in obtaining sediment records added much to early knowledge of the suspended load carried by New Mexico streams. His work was the prelude to the establishment of regular sediment-sampling stations in that State. These photo- graphs present a graphic account of what we observed several times during attempts to measure arroyo flow. If a stream rises from nearly a dry condition to a depth of scveral feet "ithin 10 minutes, one might well speak of a wall of water coming down the channel. Bores as large as 2 feet in height have been seen by the authors. However, observations made during the course of this study cause us to conclude that a "wall of water" is less common than a rapid increase in stage attained through a succession of small surges or bores each a few inches in height. The rise shown in the photographs of figure 4 is believed to be typical in that the initial front does not amount to more than half of the peak depth. Jahns (949) stated that debris flows may have nearly vertical but slow-moving fronts. As he pointed out, desert floods may include all of the gradations from mudflows to nearly clear water, We believe, however, that the faster moving fronts are seldom vertical Hwalls of ,vater," but a succession of small bores as described above; Jahns' photographs also support this contention. A flood in Canada Ancha Arroyo, July 26, 1952, provided an exceptional opportunity to observe the surges or bores. At maximum flow the width was about 100 feet, mean depth was estimated to be 1 foot, and mean velocity slightly exceeded 5 feet per second. During the 5 minutes immediately preceding peak stage, a series of bores each y, to 1 foot high moved down the channel at a velocity estimatcd to be greater than that of the water itself, The approach of the third bore made it apparent that they were spaced rather regularly in time. There- after, we measured with a stopwatch the intervals between successive bores whieh were 31, 35, 34, 48, and 60 seconds respectively. Between surges the watcr ~tage decreased somewhat, as judged by sub- mergence and reemergence of a gravel bar in the channel. Furthermore, the p-eak stage was much less than the sum of the heights of the eight individual wave fron ts. The nearly constant period between five of the eight surges seems to rule out the possibility that they resulted from successive arrivals of flood peaks from different upstream tributaries. Rather the bores are a type of momentum wave associated with the hydrau- lics of the channel itself. One of our first opportunities to measure the flow in an arroyo was in a narrow and steep-walled channel cut in fine alluvium. After the stage had fallen and the channel was nearly dry, we heard a succession of "plops" which attracted attention. The source of the noises turned out to be falling chunks of bank hitting the nearly dry stream bed. It became obvious that hank caving followed the flood recession, On every subsequent opportunity, examples of undercutting and bank caving due to high velocity water were specifically sought, but never experienced. Yet, without exception, caving of wetted banks into the channel after the flood was observed. .\loreover, Oil walking any dry arroyo one will find debris fallen