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Influence of Glen Canyon Dam Operations on Downstream Sand Resources 21 mains tern transport--and thus export-has also most likely been reduced because of the elimination of the highest f100d f1ows. As a result, two possibilities exist for the postdam fine-sediment balance downstream from the Paria River. First, if the supply from the Paria River and other lesser Marble Canyon tributaries exceeds the postdam transport rate on an annual basis, then new sand inputs would accumulate in the channel and in low- elevation portions of eddies over multiple years. Such accumulated sand supplies would then be available at any time for redistribution to higher elevation sandbars through release of periodic controlled f100ds (i.e., beach/ habitat-building f10ws in the EIS; hereafter BHBF) from Glen Canyon Dam. This scenario was the conclusion reached by Howard and Dolan (1981), Andrews (1990, 1991), Smillie and others (1993), and the EIS study team (U.S. Department of the Interior, 1995) for the MLFF alternative, leading to its implementation in 1996. Howard and Dolan (1981) reached their conclusion by using an estimate for the sand contribution from the lesser tributaries that is now regarded to be about a factor of four too high (Topping and others, 2000b; Webb and others, 2000). Andrews (1990, 1991) and Smillie and others (1993) reached their conclusions by using stable sand-transport relationships, also called "rating curves." A stable sand-transport rating curve exists where there is a unique value for sand concentration for any given f1ow. This approach invokes the assumption that the upstream sand supply is in equilibrium with transport capacity. The methods and data used to reach the conclusion in the EIS are discussed further in the following section. Alternatively, if the annual mainstem transport rate (export) exceeds tributary supply (input), then systematic long-term erosion of fine sediment from the channel would be expected. In fact, this second scenario was originally predicted by Dolan and others (1974) and Laursen and others: 1976) on the basis of their early sediment-transport studies related to effects of Glen Canyon Dam on downstream resources. In order for high-flow releases to be effective at restoring and main- taining sandbars under this second scenario, controlled f100ds would need to be strategically timed to coincide with or immediately follow tributary sand inputs. These early studies predated the concept of using controlled f100ds to restore eroded sandbars; hence, their estimates of sand transport in the postdam era could only result in net export of new sand inputs and continued erosion of existing sandban of predam origin. l\'fore recent evi- dence presented in the following section further supports the conclusion that this second scenario prevails under the current reoperaling strategy and that this situation is leading to systematic, long-term erosion of fine sediment from the channel bed and eddies of Marble and Grand Canyons. On the basis of existing data, it is still uncer- tain whether or not strategically timed managed floods can restore and maintain eroded sandbars by using only the limited and infrequent tributary-derived sand that enters the river below the dam. Recent Findings The Paradigm of Sand Transport and Storage Used in the 1995 Environmental Impact Statement The EIS concluded that sand would accumulate over multiyear timescales in the channel of the Colorado River in Marble and upper Grand Canyons during MLFF powerplant releases in all but the highest release years (U.S. Department of the Interior, 1995). The basis for this conclusion was the assumption that the relationship between the water discharge and sand transport in the Colorado River did not change substantially over time. This assumption was used because sediment-transport data collected in the postdam Colorado River were sparse. Prior to the early 1970s, suspended-sediment con- centration was measured on a daily basis at the three USGS gaging stations that are critical to constructing a sand budget for Marble and Grand Canyons: the Paria River at Lees Ferry, the Little Colorado River at Cameron, and the Colorado River near Grand Canyon. The sediment sampling program at the Colorado River near Grand Canyon gaging station began in October 1925; the daily sediment sampling programs at the Paria and Little Colorado Rivers began in October 1947. The Little Colorado River sediment record was discontinued on September 30, 1970; the Colorado River sediment record at the Grand Canyon gaging station was discon- tinued on September 30, 1972; and the Paria River sedi- ment record was discontinued on September 30, 1976. Thus, the only postdam period of overlap between these stations that could be used to construct a sand budget was the period from closure of the dam in March 1963 through September 30, 1970. Furthermore, no post- dam sand-transport data were collected within Marble Canyon during this early period. To fill this data gap, the USGS began a program of quasi-daily sediment sampling on the major tributaries to the Colorado River (that is, the Paria River, the Little Colorado River, and Kanab Creek) and at five locations on the mains tern Colorado River in Marble and Grand Canyons (Garrett and others, 1993). On the tributar-