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t I Figure 5. The line of hardwood vegetation marks the pre-dam flood level (120,000 efs?). The lower terrace is at the present 15 feet. The mean daily high dis- charged from the dam is about 20,000 cfs, and the daily low is 4,600 cfs, with extremes ranging from 2,000 to 27,000 cfs. Discharge during holiday,> and weekends is low in response to decreased power demand. Arrival of peak flow downstream is, of course, delayed because of the finite water velocity. (River guides, who carefully take these daily fluctuations into ac- count because most major rapids are less dangerous during higher water, plan their trips to avoid fol- lowing the weekend low water as it proceeds downstream.) The effect of the Glen Canyon Dam on the Colorado's sediment load has also been dramatic. At Lees Ferry, the median suspended-sedi- ment concentration has been re- duced by a factor of about 200 (Table 1). Farther downstream, however, there is less reduction be- cause of additional sediment from tributaries and from the continuing erosion of pre-dam terraces and of I, high-water mark (24,000 efs). (Photo by Robert Dolan.) the channel bed; at the gauging station near Phantom Ranch the factor of reduction is about 31/2. Changes in the hydraulic regime of the Colorado have markedly changed the alluvial morphology and the vegetation patterns along the river. The extensive pre-dam flood deposits have been eroded di- rectly by the river and by the seep. age of groundwater from terraces during daily low water. In many locations lateral erosion stops after the exposure of coarse fluvial grav- els, fan deposits, or talus, which under present controlled discharges resist movement (Fig. 7). Even where the river deposits are not protected from lateral erosion by coarse debris, a dynamic equilibri- urn may be reached where episodes of deposition and erosion become roughly balanced. Photographic comparisons of pre-dam and post- dam beach morphology provide ex- amples of both marked erosion (Fig. 8) and nearly indistinguisha- ble change. In the fall of 1972 and again in the spring of 1973, floods of the Little Colorado River produced discharges into the lower canyon of 34,000 cfs. Sediment contributed from this tributary and from bed scour built up 1 to 3 feet of fine sand and silt on the fluvial deposits not inundat- ed by post-dam discharges of the main branch of the river (Fig. 9). This resulted in the lateral growth of bars and terraces by as much as several tens of feet. During the 1973 float trip, we saw that these flood deposits were being extensively eroded. A long-term balance be- tween erosion and deposition is thus more likely to be established along the lower canyon, where the accumulated contribution of sedi- ment from tributaries and from the continued erosion of canyon terraces is greatest. The terrace and eolian deposits above the present high-water level (Fig. 4, Zones B and C) are primar- ily modified by the wind. Much of this sand will eventually be lost ei- 1974 July-August 397