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the valley and the use of agriculture by wildlife. Chapter 5 discusses human uses of the valley, other than agriculture, and summarizes effects on the fauna of the system. Chapter 6 documents recent trends in riparian and marsh habitats on the lower Colo- rado River. Chapters 7-12 describe the floral and faunal communities on the lower Colorado River. Chapter 7 provides a survey of the major plant species (including algae) found in both aquatic and terrestrial habitats. Similarly, Chapter 8 surveys the most important invertebrate species and communities again in both aquatic and terrestrial habitats. The vertebrate groups are surveyed separately, with Chapter 9 for f ish, Chapter 10 for amphibians and reptiles, Chapter 11 for birds, and Chapter 12 for mammals. Use of habitats is detailed for all vertebrate species or groups of species wherever data are available. Species of special concern and those threatened or endangered are treated for each floral and faunal group in their respective chapters. Chapter 13 provides an assessment of the health of the present-day Colo- rado River ecosystem. In this last chapter we describe means by which some elements of the natural system may be maintained or reestablished. We end with what we see are the pros- pects for the immediate future of the Colorado River ecosystem, given the present trends in management. 1.1 STUDY AREA DESCRIPTION The study area is the lower reach of the Colorado River, which flows 453 km (281.5 mi) north to south from Davis Dam, near Bullhead C?ty, AZ, to the Mexico-United States International Boundary at San Luis (Figure 1). The Colorado River originates from two main branches, the Green and the Grand (Colorado) Rivers, which drain the eastern Great Basin and the southern Rocky Mountains, respectively. The river's waters travel some 2,736 km (1,700 mi) and drop over 4,267 m (14,000 ft) in elevation before empty- ing into the Gulf of California in Mexico. Between the river's origins and the delta, the Colorado River forged many gorges and canyons, the most famous of which is the Grand Canyon. The lower Colorado River, below the Grand Canyon, establishes the present-day boundary of Arizona with Nevada, California, and Mexico. This lower stretch flows through a level and rather broad valley (Figure 2). Historically, it reached its mouth in a vast delta of alluvial silt beds, marshes, and forests. Today, the lower Colorado River encounters a series of major obstacles (Figure 3). In Arizona, the river flows through Glen Canyon Dam near the Utah-Arizona border, then through Hoover Dam below the Grand Canyon, and through a series of lesser dams to the south. Virtual- ly all of the Colorado River's water is allocated and used by the seven states (Colorado, Wyoming, Utah, Nevada, New Mexico, Arizona, and Cal- ifornia) that compose its watershed basin. The controlled flows rarely reach the Mexican border below Yuma.. AZ, and, until recently, little or no water has reached the delta in three decades. Although this region has been markedly altered by the placement of dams, an understanding of the natural events that shaped the floodplain is essential to understanding its present-day plant and animal life. Two noteworthy characteristics of the Colorado River were largely respon- sible for floodplain formation. One was the unusually large load of sedi- ments carried by the river, contribut- ing both to the erosive actions of the current and to the deposition of large 2