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lower Colorado River mesquite land were cleared north of Ehrenberg by the end of 1984 for conversion into agri- cultural production. In addition, some of the most important screwbean mesquite habitats are being cleared presently for agriculture, and also for new trailer-recreational vehicle parks. Channel straightening and armoring was completed along most of the lower Colorado River by the U.S. Bureau of Reclamation to increase the efficiency of water transport and to reduce riverbank erosion. Hydrologically, channel dredging (or deepening) lowered adjacent water tables, which effectively drained most backwaters along the lower Colorado River. Ces- sation of floods precluded development of new backwaters. Finally, dredge spoil was deposited in backwaters to decrease surface area and retention time of water (Minckley 1979). All these activities have decreased the amount of circulating organic material in aquatic habitats that would be available for primary productivity. Ironically, the most recent cause of vegetational change along the lower Colorado River is the same factor that was most essential to the continuing health of the entire system, that is flooding. Before dam construction natural floods typically lasted only a few months, whereas recent high water releases from dams may last for 12 or more months. After 1935, the river had not overflowed its prescribed channel until the summer of 1983, when water releases from dams exceeded any previously recorded controlled flows. The long duration of high flows during 1983, 1984s and again in 1986 resulted in the death of most of the remaining cottonwoods along the river (Figure 21). Cottonwood and mesquite are highly intolerant of long-term flood- Ing, whereas willows are considerably better adapted to long-term inunda- t1on. Native plant regeneration is limited by timing of the flood and by high soil salinity, both which now favor saltcedar establishment. Even though some regeneration of cotton- woods and willows has occurred many more hectares have been lost. The recent floodwaters have also covered many hectares of emergent vegetation with sediment and debris, while other marshes have been totally scoured of their vegetation. Some marshes bene- fit temporarily from inundation, how- ever, because emergent plants regen- erate and spread quickly on new silt beds covered by shallow water. This aggradation of material eventually allows the reestablishment of ter- restrial vegetation, most of which probably will be saltcedar. Much of the submergent vegetation has yet to recover from recent flooding. 2.3 FAUNAL CHANGES Two animal groups, fish and birds, have shown dramatic changes in as- sociation with increased river manage- ment on the lower Colorado River since the mid-1800's. Some changes also have occurred in other faunal taxa. Native fish and bird species have declined or have been extirpated, while many introduced species of fish and both introduced and native species of birds have increased. The native fish fauna of the lower Colorado River consisted primarily of nine species, all but one of which are presently extirpated from the main- stream or are extremely rare (Minckley 1979). Three of these species were essentially marine and all but one, the striped mullet (Mug-LL ce halus), have been cut off from the river by Morelos Dam downstream of Yuma. Five species compose a group of big-river fish, all of which have declined to near extirpation, that disappeared from the river between the 1950's and 26