Fine-Sediment Habitats
Fine-Sediment Storage Monitoring: Individual sand bar data collected from 1990 through fall 2001, show that sand bars in the actively fluctuating zone (8,000 to 25,000 cfs), and above the 25,000 cfs stage within Marble Canyon (river miles 0-61) have continued to decline since 1990, despite bar restoration gains achieved by the Beach/Habitat-Building Flow test of 1996, and peak power-plant test flows released in November 1997 and May and September 2000. Although high-elevation sand bars (above 25,000 cfs) below river mile 61 (Grand Canyon) appear to be in somewhat better condition in 2000 versus 1990, than bars in Marble Canyon, deposits within the actively fluctuating zone continue to show decline throughout the ecosystem. The sand-bar time series (1990 through 2002) suggests that the long-term fate of beaches in the upper, critical reaches of the ecosystem will likely be in continued decline under current ROD operations. Beach data collected in fall 2003 show decline in bar conditions at many sites within the first 100 miles below the dam. The most probable reason for the continuing decline of sand bars appears to be related to depletion of the ecosystem’s sediment supply. This trend might be reversed if new fine-sediment inputs from tributaries can be managed more strategically using combinations of power-plant operations and BHBF’s following tributary floods. Declining beach trends correlate with the findings of the sediment mass-balance project that indicate that new sand inputs from tributaries are transported downstream relatively quickly rather than being retained throughout the river channel and periodically re-deposited on diminishing bars.