230 DRAINAGE BASINS. 



An exception to the above conditions might be noted where a pre- 

 cipitous granitic drainage basin is expanded at the mouth of its can- 

 yon into a valley which, in turn, contracts at its outlet. This vaUey. 

 during a geologic age, becomes filled with gravel and boulders. When 

 a storm occurs on a drainage basin of this class, and the flood is pro- 

 jected upon these gravel beds, it is rapidly absorbed. The same ab- 

 sorption of irrigation water occurs on t*ese valley lands. Slow perco- 

 lation, or underflow, then follows towards the mouth of the valley, 

 which has been more or less filled with water. At the lower narrows 

 the regulated water again appears at the surface and produces a stream 

 of great constancy. Even under this condition the forest cover of a 

 drainage basin is important, owing to the fact that a constant flow 

 is desired at the point where the water leaves the upper portion of 

 the basin, and because floods often are discharged too large In volume 

 to be wholly absorbed, and they in part pass completely over the gravel 

 bed, escaping through the lower narrows, to be permanently lost. These 

 conditions exist in the Los Angeles, the San Gabriel and the Santa Ana 

 rivers, in Southern California. 



A striking example of the output of a barren drainage basin is 

 shown in the diagram of Queen Creek, Arizona, for the year 1896. In 

 this diagram the vertical axis indicates volumes discharged, and the 

 horizontal axis indicates time. This stream discharges only in vio- 

 lent freshets, recurring usually as great flood-waves, subsiding almost 

 as rapidly as they arise. By making from two to three current-meter 

 measurements of each of these freshets, and keeping an hourly record 

 of the gauge height, the discharge can be approximated. The floods 

 are usually not to exceed twelve hours in duration. During the greater 

 portion of the year the channel is entirely dry. Queen Creek rises in 

 the mountains to the southwest of Phoenix, and flows in a general 

 southwesterly direction, losing itself in the desert north of the Gila 

 River Indian Reservation. The area of the drainage basin is 142.5 

 square miles, of which 61 per cent is above an elevation of 3,000 feet, 

 and 39 per cent below that elevation. The annual discharge is ap- 

 proximately 10,000 acre-feet. The basin is almost entirely bare, there 

 being a few pinon trees and very little brush or grass. The follow- 

 ing table of discharge for the year 1896 for Queen Creek is taken from 

 the Eighteenth Annual Report of the Geological Survey, Part IV, Hydro- 

 graphy. It represents a typical year's output: 



Estimated monthly discharge of Queen Creek at Whitlow's Arizona. 

 Drainage area 143 square miles. 



