Prior to human development in the West, rivers flowed freely. Flows in the Colorado River varied greatly with season, with snowmelt runoff from the Rocky Mountains resulting in annual high flows (Topping et al. 2003). The large sediment loads historically found in the Colorado River, estimated to average 160,000,000 tons passing Yuma annually (LaRue 1916), have since caused Laguna Dam (fig. 23) near Yuma, Arizona, completed in 1905, to silt in almost immediately. Even in a managed system, unpredictable unmeasured tributary inflow into the Colorado River below Hoover Dam accounts for approximately 96,000 acre-ft of water annually, with about 62 percent of that flow from Arizona tributaries, including Sacramento Wash, the Bill Williams River area, Bouse Wash, Tyson Wash, the Dome Rock-Trigo-Chocolate Mountains area, and the Gila River area (Owen-Joyce 1987). Grinnell (1914) provided an account of mammals and birds of the Lower Colorado River Valley, which provides a benchmark from early in the development of the river. Riparian habitat became established following seed deposition on surfaces where sufficient soil moisture maintained the vegetation to maturity. As we see today, large numbers of seeds germinate but few plants survive to maturity to provide seed to repeat the cycle.