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<br />312
<br />
<br />BULLETIN OF THE ASSOCIATION OF ENGINEERING GEOLOGISTS
<br />
<br />Table I. Altitude, drainage area, date and type of event, and slope-area di~'Charge of sites investigated (DF, debris flow; WF,
<br />waterflood with large sediment load).
<br /> Drainage Slope-Areal
<br /> Area, in Discharge, in
<br />;ite Altitude, Square Type Cubic Meters
<br />Num- in Meters Kilometers Date of of per Second
<br />ber Location (ft) (mi2) Event Event (ft3/S)
<br /> Red Dirt Cr~ek tributary 2,475 6.6 Unknown DF Unknown
<br /> (8,120) (2,5)
<br />2 Lucky Gulch near Dotsero, Colo. 1,960 ),5 1976 WF 142
<br /> (6,430) (1.4) (5,010)
<br />3 South Halfmaon Creek tributary near Lead- 3,415 0,65 1977? DF 265
<br /> ville, Colo. (IUOO) (0,25) (9,360)
<br />4 South Fork Dutch Creek tributary near Red- 2,790 ],6 1977 DF 28
<br /> stone, Colo. (9.150) (1.4) (990)
<br />5 East River tributary near Crested Butte, Colo. 2,930 0.43 1977 DF 157
<br /> (9,610) (0.17) 15,540}
<br />6 Skyrocket Gulch near Ouray, Colo. 2,)40 2,6 1923 WF 42
<br /> (7,680) (1.0) 1951, 1971 DF (1,480)
<br /> Unknown
<br />7 Cornet Creek at Telluride, Colo. 2,770 6,2 1969 DF 28
<br /> (9,080) (2.4) 1914 DF (990)
<br /> Comparable in
<br /> magnitude
<br /> to 1969
<br />
<br />1 Slope-area estimates are not valid when applied to debris flows.
<br />
<br />of poorly sorted debris are available on side slopes
<br />for mobilization. Thus, the rate of colluvium for.
<br />mation in small mountain basins, which is con-
<br />trolled by rates of weathering, also is an important
<br />factor leading to debris flows. Debris flows have
<br />even been considered to be an ordinary and ex-
<br />pectable geologic process in small basins in parts of
<br />the western United States (Beaty, 1974; Scott,
<br />1971).
<br />The data in Table I and other published and un.
<br />published geomorphic and hydrologic information
<br />we have collected suggest that an elevation between
<br />approximately 2, ISO and 2,450 m (7,000-8,000 ft)
<br />separates two environments in the Colorado Rocky
<br />Mountains (McCain and Jarrett, 1976) in which hy.
<br />drologic processes may differ. Below this elevation
<br />range, intense rainfalls over large areas are frequent
<br />and may cause large waterfloods with high sediment
<br />loads (mountain torrents), such as occurred in 1965
<br />along tributaries of the South Platte River and in
<br />1976 in the Big Thompson River and tributaries.
<br />Above this range in elevation, intense rainfall is less
<br />frequent than commonly believed, and debris flows,
<br />as well as waterfloods occur in response to rainfall.
<br />
<br />GEOMORPHIC AND SEDIMENTOLOGIC
<br />EVIDENCE OF DEBRIS FLOWS
<br />AND WATERFLOODS
<br />
<br />Several kinds of geomorphic and sedimentologic
<br />evidence remain in small mountain basins following
<br />waterfloods and following debris flows that can be
<br />used to differentiate the two processes. This evi-
<br />dence includes: (I) the presence or absence of
<br />coarse, poorly sorted leaves and terminal lobes bor.
<br />dering the channel; (2) sedimentology of the depos.
<br />its; (3) the extent of damage to adjacent vegetation
<br />on fans at the mouths of basins; and (4) the extent
<br />of ground.litter disruption below high. water marks.
<br />Analysis of records from a gaging station down-
<br />stream from a basin also can be used to distinguish
<br />a debris flow from a waterflood. The seven areas
<br />shown in Figure 2 will be used to illustrate some of
<br />the geomorphic and sedimentologic evidence that
<br />can be obtained in small mountain basins to help
<br />differentiate between waterfloods and debris flows.
<br />Following a waterflood, landforms and resulting de-
<br />posits in small basins are different from those reo
<br />suiting from a debris flow, Study of these landforms
<br />and deposits commonly will enable one to interpret
<br />
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