ABSTRACT
Native riparian vegetation has changed considerably along the mainstem of the Colorado River between Glen Canyon Dam and Lake Mead since the closure of the dam. Old high water line (OHWL) plant species are in decline despite the shift some species have made into new, lower zones of the riparian area. Plants and sediment substrates directly adjacent to the river have been subjected to much less scour and desiccation with the post-dam hydrograph, and many woody species have been able to colonize much larger areas relative to pre-dam conditions. Novel communities such as return current channel marshes have developed in the canyon due to the lack of scour in backwater habitats. Overall, there has been a significant increase in the areal extent of riparian vegetation along this section of the Colorado River.
The 1996 test flood was expected to scour existing riparian and marsh vegetation. Monitoring conducted after the flood revealed that vegetation was buried rather than scoured, and was able to recover to pre-flood levels within six months. Flows the size of the test flood, 45,000 cfs (1,274 m3/s), are inadequate to achieve current vegetation management goals, and will thus need to be modified if vegetation management goals are to be met.