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<br />EPHEMERAL STRE~fS <br /> <br />3 <br /> <br />As one approaches Santa Fe from the west, it appears <br />that the Sangre de Cristo Mountains are abutted by a <br />single relatively broad remnant of an erosion surface <br />cut on poorly consolidated sand and gravel of the Santa <br />Fe formation of Miocene and Pliocene age. From the <br />top of any of the lava-capped mesas near the Rio Grande, <br />one obtains a splendid view of this surface which slopes <br />gently upward to the east toward the mountains. <br />Actually, the plain, which from afar appears smooth, <br />consists of several erosion surfaces differing but little <br />in elevation. Furthermore, close inspection shows that <br />the relief is in fact greater than it appearcd from a more <br />distant view. Rolling hills, dissected by gullies and <br />rills and interlaced with sandy, flat-floored washes, are <br />nearly everywhere at hand. The gencral appearance <br />of streams in the area studied can be visualized by in- <br />spection of the phot,ographs in figures 2 and 3. (For <br />definition of strcam order see p, 16.) The network of <br />drainage channels ramifies upstream into increasing <br />numbers of successively smaller gullies and rills extend- <br />ing almost to the divides. <br />As else\~...here in arid regions, these channels present, <br />a variety of forms. They range from tiny rills biting <br />back into mesa escarpments to deep trenches or f1ume- <br />like arroyos incised in otherwise flat alluvial valleys. <br />Despite these striking differences in appearance, they <br />all have certain common characteristics besides the <br />ephemeral nature of their flow. First, vegetation in or <br /> <br /> <br />FIGURE Z.-Channel of Arroyo Caliente, a small tributary to Arroyo de los Frijoles <br />near Santa Fe, N. Mex. The channel shown i8 typical ola filth-order stream in the <br />a.reastudied. <br /> <br /> <br />FIGURE 3.-Channel of Arroyo San Cristobal Just below Highway 41, I mile south of <br />Galisteo, N. ::'vlex. This stream is of 10th order. Mudballs in the channel bed <br />attained a diameter of 2H fee-t. High.water marks indic~ted recent nows flooded <br />thf' .,,{mOWS in upper right and nearly reacbed the top of the yertical bank seen just <br />to the left of the phnetll.ble. <br /> <br />along the channel is so sparse that it exerts almost no <br />influence on the form of the channel. Second, because <br />large amounts of fine debris are readily available for <br />transportation both in interstream areas and in the <br />channel itsclf, the sediment concentration during flows <br />greatly exceeds that of strcams in more humid areas, <br />Third, downcutting appears to proceed slowly after the <br />initial development of gullying in a given reach. In a <br />developing arroyo system, channels quickly aehieve <br />maximum depth and thenceforth little change in depth <br />occurs despite a considerable thickness of easily removed <br />material. <br />An arroyo in the Southwest discharges water only <br />when a moderately heavy rain' falls on the drainage <br />basin. This is typically a summer phenomenon since <br />flow-producing rain falls only from thunderstorms. <br />VVinter rains arc of too low intensity to provide surface <br />runoff and for this reason arroyos practically never flow <br />in win tel' . <br />Summer thunderstorms in K ew Mexico typically pro- <br />duce rain over 5 to 50 square miles, but the larger cov- <br />erage ordinarily results from movement of the storm. <br />The storm itself covers, on the average, about 10 square <br />miles. Depth-area curves of thunderstorm rainfall in <br />New Mexico published by Leopold (1942) show that <br />usually not more than 3 to 4 inches of rain falls over 3 <br />square miles and the amount decreases to 1 inch over <br />50 to 100 square miles. It was our experience that <br />fIltsh flow in arroyos seldom occurs if the rainfall at the <br />