4 BULLETIN" 61, U. S. DEPARTMENT OF AGRICULTURE. 



storms, their distribution, and frequency of occurrence. As might be inferred, the 

 ■weather reports -would not reflect this phase of the precipitation to any marked extent. 

 The mean annual temperature and the highest and lowest temperatures are given 

 in Table III (See Appendix). It is worthy of note that the temperature range in the 

 basin region is great. Extreme cold often prevails in the northern part and extends 

 well down to the south. Extreme summer heats are characteristic of the southern 

 portions and extend well up to the north. As a consequence of this, rock disintegra- 

 tion would be a not inconspicuous feature of the higher mountains. 



TOPOGRAPHY. 



The dominant bounding ranges of the basin region are the Sierra Nevadas on the 

 west and the Wasatch on the east. The area between these ranges may be considered 

 as a plain iatersected by mountain ranges of a predominantly north and south trend. 

 The plain, which is really a great system of more or less connected intermountain 

 valleys, maintains its elevation of between 4,000 and 5,000 feet altitude over practi- 

 cally the entire northern half of the region. The northern half contains three of the 

 main drainage basins — the Bonneville, the Lahontan, and the Oregon Lake basins. 

 These basins are all close to or within the 4,000-foot contour. The south-central half 

 of the plain slopes gradually to the south, reaching two points of maximum depres- 

 sion — Death Valley on -ftie southwest and Las Vegas Valley on the southeast. If we 

 consider the Salton Lake area as a portion of the Great Basin, we have another point 

 of low depression in the Salton Sink. The principal river within the basin is the 

 Humboldt. This river flows across Nevada and feeds Humboldt Lake, in the Lahon- 

 tan basin. Of minor importance are the Quinn, Amargosa, Reese, and White Rivers. 

 From the Sierras and the Wasatch Mountains many important streams feed the lakes 

 lying in the Bonneville and Lahontan basins and along the base of the Sierras. Many 

 minor streams flow from the short, steep canyons of the higher moimtain ranges of the 

 basin. 



The mountains of the basin region are in many instances characterized by steep 

 scarps on either or both sides. Short, steep canyons cut to the summits are the rule. 

 Only in a few instances are gently rising slopes to the higher summits to be found. 

 The topography of the mountains belongs to an intermediate rather than a mature or 

 juvenile type. 



The valleys are wide and often of great north and south extent. Fringing the 

 valleys are alluvial fans or cones. They are less noticeable in the north, but become 

 conspicuous in the south, where they reach enormous proportions in the Death Valley 

 region. 



An attempt has been made to determine the proportion of mountain and inter- 

 mountain area. The Sierra Valley, Reno, Wadsworth, and Carson topographical 

 sheets were measured and the areas occupied by mountain, outwash slope, silt, playa, 

 and lake determined l^y planimeter measurements. A more or less arbitrary diAdsion 

 was made between mountain and outwash areas, and between outwash and silt areas. 

 Outwash areas include the alluvial fans or cones fringing the steep slopes of the moun- 

 tains. Where the contours indicated a 2° to 4° slope, the beginning of the silt area 

 was assumed, while the blue dotted line upon the topographic sheets surrounding the 

 lowest area of an intermountain space was taken as the playa area. Similar measure- 

 ments were made upon the topographic sheets of the Amargosa River. The results 

 of these measurements, as well as those made in the Owens River Valley, are given 

 in Table IV (Appendix). Figure 1 graphically illustrates the comparison of the 

 areas measured, with the exception of the Owens Valley area. The measurements 

 given may be taken to represent a close approximation to the conditions within the 

 basin region. The mean of the measurements of the Carson and the Amargosa region 

 is: Mountain area, 48.3; outwash slopes, 19.1; silt area, 26.8; playa and water area, 

 5.5 per cent. The mean may be taken to represent approximately the basin region. 

 The figures may be interpreted to mean that over approximately one-half of the b.asin 

 region erosion is active, while on the remaining half deposition is taking place, greatest 

 in amount on the outwash slopes and least in the playa and flat portions of the inter- 

 mountain areas. The material constituting the outwash slopes is, in the main, 

 coarse and angular. It is itself more or less subject to erosion. The fine silt and sand 

 coming from the mountain areas, as well as the eroded material of the outwash slopes, 

 finds its way into the playa areas. 



GEOLOGY. 



An extensive rcAiew of the geology of the 1 asin region would be out of place here. 

 Briefly, all of tlie geological divisions, with the exception of the prc-Cambrian, Per- 

 mian, and Cretaceous, are to be fourid. For our purpose we may consider these geo- 

 logical time divisions in three groups — pre-Tertiary, Tertiary, and post-Tertiary. 



