58 University of California Publications in Zoology [Vol.12 



the year. (2) There is au enormous increase in the volume of the 

 river at the annual period of high water (see Newell. 1904. pp. 123- 

 161). 



The sediment load varies from one-tenth to two and one-tenth per 

 cent, by weight, according to the rate of the current. The latter varies 

 from a maximum of two and one-half miles per hour at low water 

 up to even seven miles per hour at flood time in parts of the channel 

 where conditions are most favorable. While it is probable that the 

 Needles-to-Yuma section of the Colorado River has about reached 

 grade, the extraordinary supply of sediment always fed from above 

 and the ever fluctuating volume of water, result in a continued pro- 

 cess of deposition and erosion, going on simultaneously in different 

 parts of the river's channel. At low water deposition exceeds erosion, 

 but at flood time the increased transporting power of the river results 

 in removal of large masses of soil from varying parts of the river 

 bottom. 



The flow of the river varies from 4.000 to 100,000 cubic feet per 

 second. The time of lowest water is in midwinter, that of highest 

 flood, in June, at the time of melting snow among the sources of 

 the Colorado River, in the Rocky Mountains. The period of high 

 water is of short duration, about May 15 to July 1. while throughout 

 the year fluctuations of less extent are liable to occur at any time. 



The river's course is of two markedly different types: where it 

 flows between rock walls among hills the channel is of fixed location 

 and usually deep and narrow ; in the large valleys the river may be 

 broad and shallow, but at any rate of constantly shifting channel. At 

 The Needles the river passes between high hills and the narrow chan- 

 nel thus formed has two sharp bends in it. Such a gorge is locally 

 called a "box canon, " and at certain stages of the river is not without 

 danger to small boats because of whirlpools and jutting reefs of rock. 

 At low water the voyager is astonished at the elevation of the highest 

 water marks on the rock walls above the surface of the river at the 

 low stage. 



There is good reason for believing that the river is now aggrading 

 its channel considerably above the apex of the delta where, of course, 

 this process is certainly in progress. At any rate, sedimentation has 

 formed broad flood-bottoms at intervals along the river's course. 

 These, large or small, are very similar in physical and floral features. 



The most extensive one of these flood-bottoms is sometimes known 

 as the "Great Valley of the Colorado." The settlements of Blythe, 



