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<br />- 20 - <br /> <br />a water content of only 25% to 30% by weight (Sharp and Nobles, 1953, <br />p. 552). The same authors cite typical surge front velocities in <br /> <br />the range of 1 to 14.5 ft./sec., and an average velocity of 9.4 ft./ <br /> <br />sec. The gradient of these flows varies from approximately 9 degrees <br /> <br />within the contained canyon section to less than 1 degree at its <br /> <br />outermost limits. It should also be noted that the initial high <br /> <br />kinetic energy is probably attained on the steep upper slopes of the <br /> <br />watershed, and that control measures taken in the lower areas may be <br /> <br />ineffective or short1ived, or both. <br /> <br />(2) Because of the above described conditions, we are virtually <br /> <br />certain that conventional velocity dissipating ponds or other struc- <br /> <br />tures on the existing stream channels will be completely ineffectual <br /> <br />in watersheds subject to voluminous mudflows as well as normal stream <br /> <br />flooding. If there are any doubts concerning the volume and composi- <br /> <br />tion of mudflow debris, the very recent deposits on lower Slate Creek <br /> <br />should be inspected (figures 3, 5, 6, 7, 8). The bulk of this debris <br />is of boulder size with a matrix of fine material, and on the top sur- <br /> <br />face of the flow near the apex of the mudflow sheet, there are numerous <br /> <br />blocks of Mesa Verde sandstone as large as five feet on a side (figure 3). <br /> <br />This not only shows the great energy and competence of the mud floods, <br /> <br />but also indicates that much of the coarse debris has come from the <br /> <br />upper cliffs of Gallo Bluff, where the massive beds of sandstone crop <br /> <br />out. Again, we wish to stress our opinion that because of the particular <br /> <br />characteristics of alpine-type mudflows, any conventional "flood control" <br /> <br />approach to controlling them or minimizing their impact will almost <br /> <br />certainly be doomed to failure. Even defensive or control measures that <br />