6 LAKES OF NORTH AMERICA. 



Grand Coulee, and near the present site of Coulee City, it plunged over 

 a precipice about two hundred feet high, and formed a cataract of the 

 nature of Shoshone falls, Idaho, but rivaling Niagara in grandeur. Two 

 basins Avere excavated in the rocks at the base of the falls, Avhich were 

 left as lakes when the glaciers retreated and the Columbia returned to its 

 old channel. These lakes still exist although desert shrubs grow on the 

 brink of the precipice over which the waters of the flooded and ioe-laden 

 river previously thundered. Each of the lakes is by estimate a mile 

 long and half a mile broad, and of considerable depth, as is shoAvn by the 

 dark blue color of their waters when seen from the crest of the encircling 

 cliffs.i 



The deeper positions of stream channels excavated during floods, may 

 be transformed into lakes when the waters subside or when the course of 

 a stream is changed. This is shown by the temporar}^ ponds remaining 

 in many humid countries during droughts when water no longer flows 

 through the customary surface channels, but is more common in arid 

 regions where the streams are subjected to still greater fluctuations. 



The basins just described are formed principally by excavation, those 

 noted below are due to deposition. 



In regions of rapid erosion, a high grade and consequently rapid 

 tributary, may bring to a sluggish trunk stream more detritus than it is 

 able to carry away. When this happens, the main stream is more or less 

 completely obstructed, and lakes may result. Basins of this nature occur 

 in the steep-Avalled valleys of the Sierra Nevada and Rocky mountains, 

 and are to be expected wherever streams have cut back their trenches far 

 into an upland and receive high-grade tributaries. 



The alluvial cones about the bases of mountains in the Arid Region 

 are frequently several miles in radius, and have a thickness near the 

 mouths of the gorges from which the material forming tliem was dis- 

 charged, of two or three thousand feet or more. When such deposits are 

 formed on the opposite side of a valley only a few miles across, they may 

 unite one with another so as to form transverse ridges and give origin to 

 basins. Alluvial cones are especially conspicuous in regions where the 

 drainage in the valleys is weak or entirely wanting, thus favoring the 

 formation of basins in the manner just described. Lake Tulare, in 

 southern California, may be cited as an example, as it is retained on a 

 l)road alluvial plain ])y material swept out by torrents from caiions in the 



' I. C. Russell, "Geological Reconnoissance in Central AVashington," U. S. Geol. Surv. 

 Bulletin, No. 108. 



