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<br />Figure 2. Modern or dredged Colorado River channel flowing through the broad <br />alluvial valley near Parker, AZ. Almost all native vegetation beyond the river <br />levees has been removed and replaced with agricultural crops. Vegetation <br />within the levees is composed primarily of exotic saltcedar (Tamarix chinensis <br />CL. pentandra]). Photo by R.D. Ohmart. <br />Figure 3. Parker Dam, completed in <br />1938, was the second major obstruction <br />to the flow of the lower Colorado <br />River. This dam, along with Hoover <br />Dam completed in 1935, dramatically <br />changed the flooding patterns of the <br />lower Colorado River. Photo by R.D. <br />Ohmart. <br />expanses of alluvial soil (Sykes <br />1937). The other was the river's <br />enormous fluctuation in water levels, <br />with an annual period of flooding <br />between 15 May and 1 July. Peak flows <br />were determined largely by the size of <br />the annual snowpack in the Rocky Moun- <br />tains, far to the north, and how <br />rapidly it melted. South of Davis Dam <br />the only tributaries of the lower <br />Colorado River are the Bill Williams <br />and Gila Rivers, both entering from <br />the east and together draining much of <br />the higher portions of Arizona and <br />southwestern New Mexico. <br />The historic channel of the lower <br />Colorado River constantly shifted <br />except where it cut through bedrock. <br />Within broad alluvial valleys the <br />4