IQWP Downstream
Fine-Sediment Mass Balance - Results of sand-transport mass-balance calculations for the period of fall 1999 through September 2000, show that sand loads passing the Grand Canyon gage, located 102 miles downstream of Glen Canyon Dam, exceeded total estimated tributary inputs; except during the period of June through August 2000 (Low Summer Steady Flow test), when dam operations were held constant at 8,000 cfs. Sand mass-balance data for October 2000 through November 2001, do show evidence of some accumulation of sand upstream of Phantom Ranch (river mile 87), in response to an approximate 1,000,000 metric ton input of sand from the Paria River in October 2000, in combination with relatively low-flow releases from Glen Canyon Dam throughout Water Year 2001. Additional sand inputs from the tributaries that occurred during September of Water Year 2002, also accumulated in the channel bed under the low-flow operations of September through December 2002. However, preliminary observations during January through March 2003, suggest that experimental fluctuating flows exported 2002 sand inputs from critical reaches above Phantom Ranch.
Overview of Suspended-Sediment Transport Monitoring - Existing management actions taken through Water Year 2003, under the Record-of-Decision (ROD) have failed to meet even the expectations contained in the Glen Canyon Dam EIS that, compared to the no action alternative, the preferred alternative would result in sand resources in the CRE increasing over time. The basic finding of the mass-balance project team is that downstream transport of new sand inputs occurs much more rapidly than was previously predicted by the Glen Canyon Dam EIS writing team (Rubin et al., 2002). The rapid export of new sand inputs measured during 1999 through 2003, from sediment-starved upstream reaches such as Marble Canyon, indicates that the ecosystem’s sand supply does not become progressively enriched over multi-year periods, except during periods when monthly release volumes are at about 700,000 acre feet or lower. If most ROD dam operations prevent new sand inputs from accumulating within the river channel, then re-deposition of new sand inputs cannot occur during occasional controlled floods, termed Beach/Habitat-Building Flows.
Such periodic releases are intended to restore and maintain sand bars that have experienced erosion since dam closure. Suggested alternatives for better conserving new sand inputs include timing the release of bar-building floods to more closely follow significant periods of sand input from tributaries. Another alternative is to schedule BHBF releases during periods when ROD operations at Glen Canyon Dam reflect below-average basin-hydrology conditions.