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<br />sedimentary structures, very poor sorting, and
<br />massive appearance, In low-gradient cbannels,
<br />debris flows typically are depositional. In steep
<br />channels, all of these types of floods can cause
<br />erosion, particularly debris flows (pierson, 1980).
<br />In addition to debris flows, streamflow and
<br />"byperconcentrated flow" occur in Grand Canyon.
<br />Streamflow typically bas a sediment concentration
<br />by weight of less tban 40 percent (pierson and
<br />Costa, 1987). Hyperconcentrated flow, as
<br />originally defined by Beverage and Culbenson
<br />(1964) and modified by Pierson and Costa (1987),
<br />contains 40 to 70 percent sediment by weigbt.
<br />Hyperconcentrated-flow deposits are differentiated
<br />from those of streamflow and debris flow by
<br />sedimentological criteria based on differences in
<br />particle-size distribution, sedimentary structures
<br />such as sligbt laminar bedding, and an overall
<br />coarse-sand, upward-coarsening texture commonly
<br />containing erratic cobbles and boulders (Pierson
<br />and Costa, 1987). Hyperconcentrated flow bas
<br />been associated with recessional flow following
<br />debris flows in Grand Canyon (Webb and others,
<br />1989).
<br />
<br />Magnitude and Frequency of Debris
<br />Flows
<br />
<br />Before 1990, three studies addressed the
<br />magnitude and frequency of debris flows in Grand
<br />Canyon, Cooley and others (1977) examined debris
<br />flows that occurred in 1966 in several tributaries of
<br />the Colorado River, including Lava Canyon and
<br />Crystal Creek (river miles 65,5-R and 98.2-R),
<br />They estimated tbe magnitude of the debris flow in
<br />Dragon Creek, a tributary of Crystal Creek (river
<br />mile 98.2-R), and inferred some frequency
<br />information from damage to archaeological sites.
<br />In an examination of aerial photography, Howard
<br />and Dolan (1981) reported that 25 percent of all
<br />debris fans in Grand Canyon had been affected by
<br />tributary floods between 1965 and 1973, In
<br />addition, Webb and others (1989) reported
<br />magnitude and frequency information for three
<br />tributaries of the Colorado River.
<br />Many researchers have described the rapids
<br />that dominate the river corridor of Grand Canyon
<br />(Leopold, 1969; Cooley and others, 1977; Graf,
<br />1979; Howard and Dolan, 1981; Webb and others,
<br />
<br />1988, 1989; Melis and others, 1994). The
<br />infrequent and episodic nature of debris flows in
<br />Grand Canyon's tributaries results in catastropbic
<br />modifications to alluvial debris fans and associated
<br />rapids over very short time periods, in most cases
<br />minutes to hours (Webb and others, 1988, 1989).
<br />Similarly, debris flows are capable of altering saod
<br />bars, commonly termed "beaches," througb burial
<br />and (or) erosion when they issue from tributaries
<br />into the river channel. Debris flows also influence
<br />the net volume of fine sediment stored in the river
<br />cbannel by forming low-velocity sediment traps,
<br />commonly referred to as eddy-complexes,
<br />upstream and downstream of debris fans. Eddies
<br />effectively trap fine sediment entering the river
<br />channel from tributaries (Schmidt and Graf, 1990),
<br />Howard and Dolan (1981) attributed
<br />aggradation on debris fans between 1965 and 1973
<br />to tributary flooding, but only generally referred to
<br />debris flow as a sediment-transport process, Otber
<br />researchers have more-fully documented the role of
<br />debris flow in the creation and maintenance of
<br />debris fans and rapids in Grand Canyon (Cooley
<br />and others, 1977; Webb, 1996; Webb and others,
<br />1988, 1989; Melis and others, 1994). On the Green
<br />River, Graf (1979) studied the effects of regulated
<br />releases from Flaming Gorge Reservoir on
<br />downstream rapids. He reported a significant
<br />increase in the stability of rapids, and predicted a
<br />trend of continuing aggradation at those sites
<br />because of reduced mean-annual discbarges in the
<br />river.
<br />Before flow regulation began in 1963, the
<br />Colorado River in Grand Canyon was known for
<br />the bigh inter-annual variability of its flooding,
<br />Periodic, large floods on the river worked togetber
<br />witb tributary rockfalls and debris flows in forming
<br />one of the world's most spectacular erosional
<br />features, The reduction of the size of tbe annual
<br />flood on tbe Colorado River since 1963 now limits
<br />the river's competence to extensively erode newly-
<br />deposited debris that continues to accumulate on
<br />debris fans. Howard and Dolan (1981) report that
<br />tbis decrease in the size of flood flows represented
<br />a four-fold decrease in the sediment-transport
<br />potential of the river. Tributaries downstream from
<br />Glen Canyon Dam remain unregulated, and their
<br />continuing debris flows remain an effective agent
<br />of change to the river corridor (Howard and Dolan,
<br />1981; Webb, 1987), As a result, the "quasi-
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