AZ vs. CA:04 California Exhibits, Vol. IV; Nos. 151-192

page 16

16 TOTt Alili-AMERICOr CA2TAI* SOME CHANNEL CHARACTERISTICS AND THE RIVER DISCHARGE. That the river required but slight assistance to make such changes as those of recent years will be understood when it is recalled that the general fall of the ground from Pilot Knob southerly to the Gulf of California is only about 1$ feet per mile, which the river in its meanderings had cut down to an effective fall of about 1 foot per mile. Toward Volcano Lake in a southwesterly direction from the same point the surface gradient is about 2 feet or more per mile, and westward in the general direction in which the Alamo flows it is nearly 3 feet per mile to the vicinity of Calexico, and thence northward into Salton Basin the country falls away 4 to 5 feet per mile. These gradients which apply to ground surface and do not represent the fall of the water surface following the meanderings of the river, will explain why the river can so readily establish a new channel when once an adequate cut or breach has been made in its west or north bank. While tiie river was on a course to the Salton Sea, 1905 to 1907, and again since 1909 on its course to Volcano Lake, the energy represented by the water flowing on the heavy gradients on these lines caused the water to cut deep into the ground. This phenomenon was accomplished by a recession of grade upstream, so that at the present time the bottom of the river bed from Hanlon downstream is lower than it had been while the river was on its original course to the gulf. While it is not possible to present conclusions in precise figures as to the amount of lids depression of the river channel, it can be stated in general terms that as far upstream as Pilot Knob, near the heading of the Imperial Canal, it amounts to 4 or 5 feet. The resulting lower elevations of the river's low waters make it more difficult to divert water into the Imperial Canal; but the carrying capacity of the river has been somewhat increased by the depression of the river bed. The river temporarily can carry more water in its channel without overtopping its banks than it could carry preceding 1909 while flowing on a direct course to the gulf. The flood menace in the upper delta reaches of the river is therefore less than it would be if the river had not changed its course, but evidence is reported that the river is now beginning to raise its bed again. The channel of the lower Colorado River in its straight reaches has a width of 600 to 900 feet. In its bends it may spread out to half a mile or more. Its bottom is a fine sand, 'which under a steady flow of water is packed hard and firm but which yields readily to the shock of waves and swirls. At its low stages, as already stated, the river has a discharge of 3,000 to 5,000 second-feet. When in flood its discharge may rise to more than 200,000 second-feet.