ABSTRACT
The Grand Canyon has been invaded by novel vegetation communities. After Glen Canyon Dam restrained historical water flows, the old scour zone was colonized by native and exotic plant species. This paper focuses on two novel vegetation communities that resulted; a tamarisk (Tamarix ramosissima) dominated riparian ‘forest’ and fluvial marshes. The marsh and tamarisk communities overlap, but I discuss them separately for their respective pros and cons. Tamarisk is an invasive weed threatening southwestern riparian communities. With limited competition, it has the ability to expand its range by prolific seed dispersal and the ability to alter soil conditions to exclude native species. Marshes, found in the fluctuation zone of the Colorado River, were rare pre-dam (Stevens et al. 1995, Kearsley and Ayers 1999). Although marsh species are mostly native, prior to the dam they were restricted to tributaries and springs where the water supply was constant. The current flow regime supplies a constant hydrograph which maintains these communities along the main stem. While experimental flow objectives target these communities, the high flow events have had little impact on them. Tamarisk and marsh communities have changed the system by increasing productivity and biodiversity in the riparian areas of the lower Colorado River.