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28 The State of the Colorado River Ecosystem in Grand Canyon Discussion and Future Research Needs Extensive research and monitoring of fine-sediment transport and sandbars since the completion of the EIS have resulted in a better understanding of the geomor- phology of the Colorado River in Marble and Grand Canyons and of the effects of the operations of Glen Canyon Dam on the river's downstream resources. Prob- ably the single most important finding of this research and monitoring is that postdam mainstem sand transport exceeds the postdam supply of sand from tributaries on a seasonal to annual basis, such that the postdam river is in an annual fine-sediment deficit (i.e., export exceeds input). This sediment deficit has resulted in a consistent downstream pattern of erosion of channel and sandbar deposits from Marble and Grand Canyons despite restric- tions on daily powerplant fluctuations required by the implementation of the MLFF alternative. The finding of an annual sediment deficit directly contradicts the critical EIS assumption that sand will accumulate on the bed of the Colorado River over mul- tiple years under the MLFF operating alternative (and minimum annual volume releases) and has important implications for the potential success of managing tribu- tary sediment inputs. It is also worth noting that the EIS conclusion resulted fundamentally from a lack of long-term records for tributary sand supply and main- stem sand-transport rates, illustrating the importance of long-term data sets in river management. A continu- ous sediment budget for the Colorado River in Grand Canyon since construction of Glen Canyon Dam, based on high-frequency measurements, likely would have resulted in a different EIS conclusion about fine- sediment dynamics below the dam, one that may have prevented the continued erosion of sandbars between 1991 and 2004. A second important finding of recent research and monitoring efforts is that during the 1996 BHBF the primary source of sand for building high-elevation sandbars was the low-elevation portion of the sandbars instead of the channel bed as hypothesized in the EIS. This scenario of building high-elevation sandbars at the expense of the low-elevation portions was repeated during the powerplant capacity flow in September 2000 (Hazel and others, in press). This process of sandbar building is supported by the finding of an absence of multiyear accumulation on the channel bed: sand cannot be transported from the bed to high-elevation sandbars because there is typically little sand available on the channel bed. Neither of these two findings supports the EIS hypotheses, but they have led scientists and managers to reassess the management strategy for sand resources within Grand Canyon. An emerging paradigm is the need to strategically time high-flow releases in order to take advantage of sporadic tributary sediment inputs, a scenario that requires greater flexibility in the annual operating plan for the dam with respect to both hydro- electric power generation and economic cost. Only immediately after these inputs is significant sand avail- able on the channel bed for transfer to high-elevation sandbars through high-flow releases. Alternatively, dam releases may be constrained following inputs for a period of time until a high flow can be released from the dam; however, during extended periods of above-average upper Colorado River Basin hydrology and high storage in Lake Powell, constraining daily operations may not be possible (see fig. I, 1995 through 1998). In the absence of high-flow releases strategically timed to redistribute tributary inputs to high-elevation sandbars, the inputs are exported from Grand Canyon in a period of weeks or months under normal dam operations, leading to continued long-term erosion of sandbars. In November 2004, this paradigm of strategically timed, high-flow releases was tested for the first time on the Colorado River. Scientists are in the process of evaluating the results of this experiment. The findings will be critical for the long-term management of fine- sediment resources and sandbars in Grand Canyon. If a management approach of strategically timed, high-flow releases, triggered by tributary inputs, is to be followed, then further research will be required to define the appropriate triggering criteria and to develop high-flow hydrographs (peaks and durations) that may optimize deposition of tributary sand inputs within eddies while minimizing export during controlled flood peaks. If strategically timed, high-flow releases are deemed inadequate for meeting the management objectives for Grand Canyon sandbars, then alternative approaches must be considered, such as further restraints on daily powerplant operations, changes in monthly volume release patterns, or sediment augmentation.