360 C. R. KEYES OROGRAPHIC ORIGIN OF LAKE BONNEVILLE 



The amount of crustal warping necessary to turn the entire system of 

 head streams of the present Snake River into Great Salt Lake basin is 

 hardly more than 200 feet. The choking of the main channel by lava or 

 the damming by slight orogenic movement could easily have directed 

 the headwaters of sucli a stream out on to the Idaho lava plains. Both 

 Gilbert^ and Gannett sliow that by changes of 100 feet the waters of Bear 

 River also might be turned into the Snake River^ or those of the Black- 

 foot and Port Neuf rivers might flow into Great Salt Lake basin. In 

 the valleys of these last mentioned streams comparatively recent basalt 

 flows have produced notable alterations in the drainage. 



GENERAL ABSENCE OF TRIBUTARIES 



It is not at all probable that between its mouth and the juncture of 

 its principal headwater streams tlie old Virgen River had any larger or 

 more lateral feeders than the inconsequential tributaries which now come 

 in from the Wasatch Mountains. During the later lake period the small 

 importance of these side streams is directly measurable by the size of the 

 deltas which were formed in the still waters. From the west there was 

 no augmentation in the river's volume. 



At the present time Gilbert estimates the capacity of these tributaries 

 at one-half the volume of Bear River. If it is assumed that in the pre- 

 lacustrine epoch the Snake River poured its waters at Pocatello into the 

 Virgen channel, these tributaries supplied less than one-fortieth or one- 

 fiftieth of the total volume of the old river. 



LOWER REACH OF OLD VIRGEN WATERWAY 



One of the most surprising drainage features of the Colorado Plateau 

 region is the canyon of Muddy Creek, which in southeastern Nevada 

 joins the present diminutive Virgen River a few miles from the point 

 where the latter unites with the Colorado River. Muddy Canyon is 

 large out of all proportion to the insignificant brook which now occupies 

 its wide, flat-bottomed floor. For a distance of 50 miles from Caliente 

 Station, where the San Pedro, Los Angeles and Salt Lake Railway enters 

 it, the bottom of this canyon is ahout 2,000 feet al)ove sealevel. Before 

 it unites with the Virgen Canyon it suddenly drops to a level about 400 

 feet above the sea. The trougli thus opens into the Colorado trench far 

 up near the summit of the inner canyon Avail. To all intents and pur- 

 poses the Muddy and Virgen troughs are wliat in glaciated regions would 

 be termed hano^ino^ valleys. Dutton^ likewise calls attention to similar 



''Mon. r. R. Gool. Survey, vol. i. ISOO. p. 210. 

 SMon. U. S. (i(M)l. Survey, vol. ii, 1S.S2, p. 227 



