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002359 4.1 Previous Projections of Long-Term Changes in the Fine-Sediment Budget The long-term fate of fine-sediment deposits was anticipated in numerous studies, and conclusions of those studies varied widely from predictions that long-term degradation of fine-sediment deposits was inevitable to predictions that smaller deposits would equili- brate with the greatly reduced post-dam fine- sediment flux. Dolan et aI. (1974) linked changes in flux to changes in sediment storage and observed that: At Lees Ferry, the median suspended- sediment concentration has been reduced by a factor of about 200. Farther downstream, however, there is less reduction because of additional sediment from tributaries and from the continuing erosion of pre-dam terraces and of the channel bed; at the [Grand Canyon gage] the factor of reduction is about 3.5. _.: Laursen et aI. (1976) described a bleak future for fine-sediment deposits: At present, the mean annual capacity of the river to carry beach-building material is about 12 million metric tons per year. The tributaries supply about 2.7 [million] metric tons of beach-building sediment per year. The difference of about 9 million metric tons per year must be obtained through scour of the bed and/or banks. ... the beaches... could be in danger of being washed away since the transport capacity of the regulated river is in excess of the amount of beach-building material being supplied from the tributaries ... How long they will last cannot as yet be estimated; certainly more than 10 years, probably less than 1000 years; but how much more or less than 100 years is a matter for continued study. Pemberton (1976) reported on the magni- tude and rate of bed degradation in the 25 km of Glen Canyon, as measured between 1956 and 1975. Pemberton (1976) argued that stability had been achieved by 1975 through bed armoring at gravel and cobble bars that act as channel controls. Howard and Dolan (1981) reached a very different conclusion than Laursen et aI. (1976) about the fate of fine-sediment depos- its. Howard and Dolan (1981) stated that: "Greatly reduced flood peaks since comple- tion of Glen Canyon Dam have decreased the turbulence generated by rapids and hence transport capacity to the extent that an aver- age of more than 1.5 m of sand has accumu- lated on the bed of [Marble Canyon and] the Upper Grand Canyon." The fine-sediment budget of Howard and Dolan (1981) was based on monthly transport data and the assumption that transport relations were stationary and did not change with time. Howard and Dolan (1981) assumed that the bed, and not eddies, was the major repository of sand. They also argued that the trend of bed aggradation at the Grand Canyon gage was representative of the entire river up- stream to Lees Ferry. Despite the projections that sand was accumulating in the riverine ecosystem, the available evidence in the late 1970s and early 1980s indicated that eddy sand bars were slowly being eroded, albeit at slow rates. Howard (1975) established profiles across 20 eddy sand bars, and resurvey of these profiles in the next few years indicated little change (Howard and Dolan, 1979). Therefore, Howard and Dolan (1981) concluded, "Dur- ing the first ten years since the dam, sandy channel banks have suffered only a very slight erosion, with individual cases of both pronounced erosion and marked deposition." Some of these sites were resurveyed in 1980 (Dolan, 1981) and two sites were resurveyed in 1982 (Beus et aI., 1982). Beus et aI. (1985) analyzed these surveys and concluded, "On balance there was slightly more loss than gain suggesting a gradual depletion of beach sand from the terraces studied." The view that reduction in transport capacity exceeded reduction in sediment supply and that sand accumulated on the 4.0 Previous Studies of Fine-Sediment Flux and Storage 21