Evolving Geomorphic Framework
Coarse-Grained Inputs and Impacts: Webb and others, of the USGS, have estimated lesser tributary contributions for both fine and coarse sediments between Glen Canyon Dam and Upper Lake Mead. They find that fine sediment inputs from the Glen and Marble Canyon reaches of the ecosystem are, on average, likely to be a factor of two greater than the estimate used by the EIS writing team in preparing the fine-sediment mass balance reported in the GCD-EIS. Although the fine sediment inputs into this critical upstream reach may be significantly higher than previously assumed, the grain-size data published in the report indicate that those sediment inputs are as fine or finer than inputs from the Paria River. This finding suggests that while sand inputs from ungaged sources are significant and worth monitoring for management purposes, these inputs likely have a short residence time in critical reaches, similar to those sand inputs derived from the Paria River (see section on Mass Balance, above). This is important information that further supports development of a fine-sediment budget for the ecosystem, as well as technical discussions about how best to conserve fine sediment inputs through dam operations. Perhaps more importantly to the ecosystem, lesser tributaries below Lees Ferry continue to input fine-to-coarse size gravel into the main channel. The implications for these ongoing, accumulated gravel inputs is still being studied, but several possibilities exist for how these deposits may influence the aquatic and terrestrial elements of the Colorado River ecosystem: 1) aggradation of the channel’s rapids leading to increased navigational challenges, but also expanded eddies where sand can accumulate, 2) aggradation of low-velocity pools and higher velocity runs, with potential changes to aquatic food base dynamics, 3) increased spatial abundance of fine gravels that effectively expand spawning and rearing habitats throughout the main channel in Glen, Marble and Grand Canyons, 4) burial and/or erosion of existing sand bars during debris flows and stream-flow floods.
A long-term monitoring program for coarse-sediment inputs and impacts throughout the ecosystem was initiated in FY 2001, although coarse-sediment inputs from lesser tributaries have been studied since 1984. The current annual monitoring effort for coarse sediment inputs is also intended to document the occurrence of periodic debris flows within 800 lesser tributaries, where and when they occur. The project is also focused on documenting how such coarse-sediment inputs alter the geomorphic framework of the river by directly impacting both sediment and non-sediment resources of the ecosystem at hundreds of locations through time under dam operations. This project represents one of the lead participants in the Advanced Conceptual Modeling project that was conducted from FY 2001 through 2003.
Summer storms of August and September 2002 and 2003, resulted in numerous localized changes in the river ecosystem’s geomorphology owing to several new debris flows and widespread tributary stream flow flooding. Extensive deposits of new gravel were deposited in the river throughout Marble and eastern Grand Canyon. The debris flows that occurred in September 2002, buried or eroded some existing campsite areas, and in one case created a significant new rapid near river mile 74.