210 



SCIENCE 



[N. 8. Vol. XLI. No. 1049 



waters have since largely disappeared through 

 diminishing water supply. The water supply 

 that maintained the larger lake, as that which 

 maintains the smaller lakes of the present 

 day, came principally from a few major 

 streams draining from the higher Sierra. Of 

 these Truckee, Carson and Walker rivers were 

 with little doubt, the dominating factors. The 

 following is an outline map showing the gen- 

 eral relation of these drainage systems. 



Outline Map Showing Truekee-Pyramid Drainage 

 System and its Former Northward Extension. 



Approximate equilibrium was maintained in 

 the larger Lake Lahontan through the balance 

 of evaporation and inflow. Evaporation varies 

 directly with the surface area of the water 

 body. Inflow is supposed to have been grad- 

 ually decreasing as the lake level was falling. 

 When, however, the waters fell to the level of 



any divide which would separate the basin into 

 two or more distinct parts, the equilibrium 

 that had been maintained for the lake body as 

 a whole would' hardly be continued in exactly 

 proportionate relations in the two separated 

 parts. Each part must have then established 

 a new relation of separate inflow and evapora- 

 tion ratio, and it is almost a certainty that an 

 overflow would for a time be established from 

 one side toward the other over the intermediate 

 divide. 



Such an overflow may have occurred over the 

 Fernley divide from the Truckee Basin into the 

 Carson Basin. The evidence of channels there 

 is not very clear. At lower elevation, how- 

 ever, such an overflow did occur from the 

 Pyramid Basin into the Smoke Creek and pos- 

 sibly beyond. The channel of this overflow is 

 indisputably clear, broad and well defined. Its 

 bottom is only YO feet above the present water 

 level of Pyramid Lake. The surface of the 

 Smoke Creek desert to the north is below the 

 water level of Pyramid Lake to-day. The 

 Smoke Creek and the more northern deserts 

 have no present perennial water supply. Al- 

 though subject to floods from winter storms, 

 they are essentially dry basins. The waters 

 that fiUed these basins during the higher La- 

 hontan stages came, with little doubt, prin- 

 cipally from the Truckee River. The chief 

 water supply of these broad evaporation areas 

 came, therefore, through the more restricted 

 basin of Pyramid Lake and flowed by way of 

 a narrow pass at the north end of Pyramid 

 Lake. As a late stage in the lake history, the 

 waters of Lahontan lowered beyond the 70-foot 

 level above present Pyramid Lake level, and a 

 distinct overflow drainage was set up out of 

 Pyramid toward the north. During all this 

 time that concentration of Lahontan waters 

 was going on, the lake in Pyramid Basin was 

 being freshened by overflow. Only when the 

 flow of Truckee River had diminished to such 

 an extent that it no longer exceeded evapora- 

 tion within the restricted basin of Pyramid 

 (including Winnemucca as in all previous 

 references) did concentration, within the 

 Pyramid Lake waters proper, begin. Estimates 

 of age based on this concentration may indi- 



