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<br />Figure 3. Beachfront along a wide, calm sec-
<br />tion of the Colorado at low water. Note the
<br />dense vegetation and eroded, silty fluvial
<br />
<br />and 220,000 cfs, in 1921. Sediment
<br />concentration ouring flood peaks
<br />varied by more than an order of
<br />magnitude, depending upon wheth-
<br />er the runoff source was the spring
<br />meltwaters in the headwaters of the
<br />system or summer thunderstorms
<br />over the Colorado plateau.
<br />
<br />The river averaged 140 million tons
<br />of suspended sediment a year be-
<br />twe'en 1935 and 1948 at the Grand
<br />Canyon Gauging Station, near
<br />Phantom Ranch (5). The total
<br />amount of sediment deposited in
<br />Lake Mead over this same period
<br />corresponded very closely to the
<br />total suspended sediment measured
<br />at the gauging station (5). indicat-
<br />ing that most of the sediment
<br />transported through the Grand
<br />Canyon and deposited in Lake
<br />Mead was suspended and that the
<br />bed load was very small. Under
<br />natural conditions the river aver-
<br />aged 0.38 million tons a day; the
<br />maximum recorded was 27.6 mil-
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<br />deposits to the left at the level of high
<br />water. The rubber raft is one of the larger
<br />motorized rigs. (Photo by Alan Howard.)
<br />
<br />A conspicuous line of hardwood
<br />vegetation (Fig. 4, Zone C; see also
<br />Fig. 5) is associated with the higher
<br />terraces. Below them, there was lit-
<br />tle permanent vegetation under
<br />pre-dam conditions, because the
<br />cycle of erosion and deposition dur-
<br />ing floods presented an unstable
<br />substrate, either uprooting or bury-
<br />ing seedlings. Furthermore, growth
<br />of phreatophytes on higher terraces
<br />was discouraged by the vertical dis-
<br />tance to the water table during
<br />summer low water.
<br />
<br />lion tons on 13 September 1927,
<br />with a discharge of about 125,000
<br />cfs (5).
<br />
<br />Most pre-dam fluvial deposits
<br />along the channel are fine-grained
<br />terraces, although bars of pebble-
<br />to cobble-sized particles do occur
<br />locally and may underlie the finer
<br />sand and silt terraces. Floods with
<br />low-sediment concentration re-
<br />sulted in the net erosion of these
<br />fine-grained terraces, whereas the
<br />occasional summer peaks resulted
<br />in deposition. This alternating ero-
<br />sion and deposition produced a
<br />time-varying, fluvial-terrace mor-
<br />phology. Measurements of pre-dam
<br />flood-terrace heights taken in 1973
<br />indicate that the terraces in the
<br />narrower portions of the canyon are
<br />18 and 30 feet above the present
<br />high water. The height of these ter-
<br />races corresponds to the pre-dam
<br />mean annual flood of 80,000 cfs
<br />(Fig. 4) and the frequent 120,000
<br />cfs peaks.
<br />
<br />Pre-dam flood terraces were depos-
<br />ited in zones of reduced river veloc-
<br />ity, such as in the mouths of tribu-
<br />tary canyons, in alcoves along the
<br />banks, on point bars in the wide
<br />sections of the river, and as narrow
<br />deposits bordering especially wide,
<br />straight stretches of the river. The
<br />most common physiographic con-
<br />text for promoting pre-dam flood
<br />terraces occurred at the rapids. Al-
<br />most all of the rapids are formed at
<br />
<br />1974 July-August 395
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