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Vegetation The vegetation in the study area has changed considerably from 1937 to the present day. Some of the basic changes were discussed earlier and are reviewed here. The site retained its floodplain character until the 1950's, even though it was diked from the river in the late 1800's. It contained much bare ground and large populations of cottonwoods. The development of irrigated agriculture on the terraces east of the wetlands created a groundwater flow system that was poorly developed prior to irrigation. This water flows generally west, erupts on the terrace escarpment and provides continual discharge to open water bodies. This flow has created and supports a ground water mound in the eastern most portion of the wetland. Open water-and emergent marshes occur in this area.. The ground water mound discharges to the west and created high water table conditions that support extensive marsh communities today and most likely drowned the cottonwoods. Six major wetland communities occur on site today. Four of these communities occupy the marsh complexes created by this ground water mound. They occur along a gradient from deep to shallow standing water and long to short-duration flooding. 1. Sago pondweed and horned pondweed marsh. The long- duration standing water areas are dominated by submerged aquatic plants, including sago pondweed (Potamocteton pectinatus), horned pondweed (Zanichellia palustris), and the green alga Chara spp. These are common and very productive communities that occur throughout the West and midwest in ponds with high alkalinity. These communities typically have peak season standing crops of approximately 300 to 500 gm Z, and the communities in the study area have similar productivities. The plants provide organic matter for the aquatic food chain, oxygen and shade for the water column and cover from predators. It is interesting to note that this community did not occur in any Moab wetlands. 2. Bulrush and cattail marsh. In areas with standing water from a few dm to 1 m in depth very productive stands of hardstem bulrush (Schoenoplectus acutus) and broad-leaf cattail (Typha latifolia) occur. These stands are monocultures in some areas. The standing crop in stands similar to this elsewhere in the West ranges from 800 to more than 2,000 gm~2, and these communities would fall into the middle range of this productivity gradient.. The stands are dense, healthy and provide important nesting sites for birds. 3. Alkali bulrush marsh. Areas with shallow standing water support one of the most extensive stands of alkali bulrush (Bolboschoenus maritimus ssp. paludosus) that we have seen in the Rocky Mountain region. It is dense, productive and supports wildlife in an astounding way. The plants produce corms approximately 2 cm in diameter along rhizomes which were favored 30