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entered the valley from adjacent desert mountain ranges. In a climate where annual rainfall averages about 5 cm (2 inches), these flows are too small to maintain permanent marshes. Although marshy backwaters and oxbow lakes were apparently limited and of short duration, they were im- portant features to aquatic life along the lower Colorado River. They pro- vided production of aquatic and semi- aquatic vegetation that supplied or- ganic matter in the form of detritus (Minckley 1979). The existence of a well-developed terrestrial riparian community along the river certainly gave rise to substantial amounts of organic materials, especially during flood events. Debris from riparian vegetation is known to play a major role in nutrient flow in low desert rivers through slow decomposition of the organic material (Rinne 1973, 1976; Bruns 1977). During the brief heyday of steam- boat traffic in the mid-1800+s, vir- tually any tree large enough or close enough to the river was burned for fuel. However, the natural resiliency of riparian vegetation ensured that the cottonwood and willow trees would regenerate. The raging floods of 1905 and 1907, however, slowed this normal rapid regeneration. By 1910, Grinnell found the willow-cottonwood associa- tion thriving in the river bottomlands once again, without mention of a con- spicuous lasting impact either by the fuel-wood cutters or the prolonged floods. Grinnell (1914:61) did, however, describe in detail the observable effects of the first dam, Laguna, on the lower Colorado River when he wrote: The water level had been raised conspicuously for at least ten miles, and we saw evidence of deepening of the first-bottom deposits and slowing of current for fully thirty miles, above the dam. The cottonwoods of the first- bottom had all been killed, evidently by the raising of the general surface around their trunks; and the mesquites and other vegetation of the second-bottom had all been drowned out, there thus being no trace of second- bottom conditions except for dead stalks. These were replaced by vast mudfIats growing up to arrowweed. All of this change, of course, involved the birds and mammals of the area affected, in addi- tion to the plant life. What Grinnell witnessed on a small scale, he could not have guessed was to be an accurate prelude to the changes in the corning decades. Two major events and their conse- quences have dictated the demise and, possibly, the eventual disappearance of the cottonwood and willow forests along the lower Colorado River (Fig- ures 19A,B,C,D). First, by 1936 Hoover Dam essentially stopped all threats of floods, except when heavy runoff from local rains brought floods from larger tributaries, such as the Bill Williams River. Farming of the rich alluvial soils increased with the cessation of flood threats. Without floods, new rich alluvial seedbeds were no longer formed and the life- history cycle of the cottonwoods and willows was irreversibly changed. In addition, lakes behind Hoover Dam, and other dams that followed, inundated thousands of hectares of riparian habitat. Of these rapid changes in the lower Colorado River Valley after Hoover Dam, Phillips et al. (1964:xv) commented: 21