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12 <br />Among the first modifications of the lower river was construction in the late 1800s and <br />early 1900s of levees to constrain flows and structures to provide bank or floodplain <br />protection, activities that continue today. Minckley (1979) estimated more than 50% of <br />the riverbank downstream from Davis Dam had been modified by riprap or other <br />armoring, or inundation. Imposition of Boulder (=Hoover) Dam on the main river in <br />1935 ended catastrophic flooding but also virtually eliminated sediment inputs from the <br />entire upper basin. These factors combined with impacts of other structures to <br />dramatically alter the character of the river, which responded by degrading its channel <br />in places where it historically aggraded and depositing sediments elsewhere. For <br />example, downcutting in the river reach below Hoover Dam from the mid-1930s to <br />about 1950 provided materials that settled in the Topock area, and today is <br />represented by the remnant Topock Marsh. Similar scenarios unfolded below other <br />dams, and these new river dynamics were countered by extensive programs to dredge <br />river and constrain it with dikes, which continue in some places today. <br />Channelized and physically constrained sections of the lower Colorado River average <br />about 150 m wide. Widths may be 60-500 m in non-impounded areas upstream of <br />Imperial Dam where the channel is not constrained. Some highly modified, de-watered <br />reaches in the Yuma area are as narrow as 10-15 m. Stream depths are greatest <br />(about 8 m) in least modified sections, and average between 1 and 3 meters elsewhere. <br />Shallowest portions occur in wide, depositional reaches at the head of impoundments, <br />where bars may be alternately exposed and watered to depths of a meter or less <br />depending upon river flow. <br />Hiebert and Grabowski (1985, 1987) quantified 10 shoreline and aquatic habitat types <br />along 71.3 km of the lower river (Table 2) through the operational reach designated by <br />USBR as Parker Division, which extends downstream from Headgate Rock Diversion <br />Dam to Palo Verde Diversion Dam near Blythe CA. They considered the upper 1/3 of <br />the reach (Parker 1) as highly modified , while the lower 2/3 (Parker II) had not been