REPORT Of Prof. A. H. THOMPSON. DEPARTMENT OF THE INTERIOR, TOPOGRAPHIC BRANCH, Washington, D. C., July 1, 1890. SIR: I have the honor to submit the following report of the work of the Topographic Branch of the Irrigation Survey during the fiscal year ending June 30, 1890. TIME AND LOCATION OF WORK. The work was continued throughout the fiscal year in the States of California, Nevada, Colorado, Idaho, Montana, the Territory of New Mexico and in office at Washington, D. C., on plans approved by you. GENERAL ORGANIZATION AND PERSONNEL. For convenience of supervision and administrative management five divisions for the prosecution of work were organized; California and Nevada constituting the first; Colorado the second; Idaho the third; Montana the fourth, and New Mexico the fifth. The California-Nevada Division was assigned to Mr. E. M. Douglas, assisted by Messrs. A. F. Dunnington, R. H. McKee, R. H. Chapman, H. E. C. Feusier, and G. H. Verrill, as assistants in charge of parties, Mr. P. V. S. Bartlett and Mr. Paul Holman as general assistants. The Colorado Division was assigned to Mr. Willard D. Johnson, with Messrs. C. H. Fitch, John W. Hays, R. C. McKinney, A. C. Barclay, R. A. Farmer, S. P. Johnson, W. S. Post, and R. B. Marshall as assistants in charge of parties. The Idaho Division was assigned to Mr. W. T. Griswold, with Mr. E. T. Perkins, jr., as assistant in charge of a party. The Montana Division was assigned to Mr. Frank Tweedy, with Mr. Jeremiah Ahern as assistant in charge of party and Mr. Frank E. Gove as general assistant. The New Mexico Division was assigned to Mr. Arthur P. Davis, with Messrs. C. C. Bassett and J. B. Lippincott assistants in charge of parties. In California and Nevada the organization consisted of one triangulation and four topographic parties; in Colorado of one triangulation, one plane-table control and six topographic parties; in Idaho of one triangulation and topographic and one topographic party; in Montana of one triangulation and two topographic parties; in New Mexico of one triangulation and two topographic parties. The work of these divisions being in a sparsely settled or entirely uninhabited region, it was necessary for the parties to subsist in camps. The organization for this purpose was nearly the same in all localities. The means of transportation usually being one large four-mule wagon for camp equipage and supplies, and buck-boards or saddle animals for the persons engaged in the map work. Usually each party employed, in addition to the regularly appointed assistants, one or two persons as traverse or rod men to assist in the field work. One cook, one teamster, and one laborer generally furnished sufficient force for camp duties. In all divisions the work proceeded, when practicable, by atlassheet areas, bounded by degree, quarter or half degree lines of latitude and longitude. These atlas sheets are named from some town or prominent natural feature within their limits and conform in scale and area to the system adopted by the Geological Survey, viz, atlas sheets when published on the scale of 2500 to represent 15 minutes of latitude and longitude each way; when on the scale of 125000 to represent 30, and on the scale 50000 1 degree. In Colorado the boundaries of a number of the atlas sheets were determined by the outlines of the drainage basin of the Arkansas River. The field work was done on twice the horizontal scale intended for publication, the relief being represented by contour lines having equal intervals, but differing on different sheets. The following table shows the locality of work, scale, contour intervals, areas surveyed, and present condition of office work at the end of the fiscal year: The work in the California-Nevada Division as organized in my last report was in full progress on July 1, 1889. To Mr. Feusier was assigned charge of the triangulation party. To the topographic party of Mr. Dunnington was assigned the Truckee atlas sheet, an area west and north of Lake Tahoe. To Mr. McKee's party was assigned an area lying south of that surveyed by Mr. Dunnington and called from its most striking feature, a sharp granitic mountain, the Pyramid Peak sheet. To Mr. Chapman's party was assigned the quarter degree lying east of Mr. McKee's work and called from a town of the same name the Markleeville sheet. To Mr. Verrill's party was assigned the quarter degree including the Carson Valley and called from the city of that name the Carson sheet. Early in August Mr. Verrill resigned and Mr. Bartlett was placed in charge of this party during the remainder of the year. All these parties completed their assigned work by November 15, and in addition the party under Mr. Chapman surveyed 340 square miles on what was named the Reno sheet, and Mr. Dunnington's party 380 square miles on the Sierraville sheet. On November 15 work in the California-Nevada Division was closed, the parties disbanded, camp equipage and field material stored, and the animals placed in winter quarters. The chief of the division, with the heads of the parties and their principal assistants, were then ordered to Washington, D. C., for office work. Topography. The area mapped lies between longitude 109° 30' and 110° 30' west and latitude 38° 30′ and 40° north. It may be said to present three great natural features: The Sierra Nevada Range, the Lake Tahoe Basin, and the Carson Valley. The Sierra Nevada Range occupies the southern and western, the Tahoe Basin the central, and the Carson Valley the eastern and northern parts. The crest of the Sierra Nevada Range stretches from southeast to northwest across the whole area surveyed, forming a sharp line between the waters flowing into the Valley of California and those flowing into the Great Interior Basin. West of the divide the slopes are gentle and below 8,000 feet in altitude, usually forest clad. This region receives an abundant rainfall and is drained by branches of the Yuba, American, Cosumne, Mokelumne, and Stanislaus Rivers. Near the eastern edge of the Pyramid Peak and Truckee sheets and at an elevation of 5,000 feet, these streams begin to cut the canyons through which they debouch into the Valley of California, but between the heads of these canyons and the divide is a region of gentler slopes descending into narrow mountain valleys through which the streams course with but comparatively little fall. East of the dividing ridge the mountain slopes are steeper, the precipitation less, the draining streams fewer, the mountain sides. unusually bare, and grow steeper and steeper until they plunge almost precipitously down into the Carson Valley. These slopes are drained by the Truckee, Carson, and Walker Rivers, all rising near the crest and flowing through valleys near the eastern edge of the work. Near the north west corner of the Markleeville sheet a spur, breaking from the main range of mountains, bears more to the east than the general trend. This spur, when near the northern edge of the Carson sheet, sweeps round to the west, becomes lower, more confused and broken, and finally, just beyond the limits of the area surveyed, merges again into the main range. Between this curving spur and the main range is an oval-shaped depression called the Tahoe Basin, having the same trend as the greater range. In its southern portion is Lake Tahoe, a body of water 21 miles in greatest length by 13 in greatest width, and having a water surface of about 190 square miles. It is surrounded on all sides by rugged mountains and high peaks; those on the west rising to 9,500 feet in altitude. The small streams draining these mountains are numerous and flow near the surface, but after the melting of the snows. decrease greatly in volume. The outlet of the lake is by the Truckee River. This stream flows first along the axis of the Tahoe Basin, then turns sharply to the north, then more to the east, and finally, after a course of 125 miles through a succession of narrow cañons and wider valleys, empties into Pyramid Lake, The northern portion of the Tahoe Basin is of lower altitude an i loss rugged than the southern and western, but yet not having. except in the extreme northwestern part of the region surveyed any considerable areas of level lands. It is drained by the Limi Truckee River and other branches of the main stream. In the region are three small lakes, Donner, Webber, and Independen the former draining into the main Truckee, and the two latter tut the Little Truckee Rivers. The only valleys are the wider openin spoken of along the main stream and its principal branek... Of the Martis and Reno, the latter being properly the northern execs a of the Truckee meadows, are the larger. The Carson Valley occupies the northern portion of the M ville and the central portion of the Carson sheets. It is separacet from the Tahoe Basin by a narrow range of mountains, the best peaks of which reach an elevation of 9,000 feet. The vallen beet has a length perhaps of 30 miles by 8 in width and rans 1 gang tion from 3,500 feet at its southern extremity to 4.30 uur t northern. To the northward it is separated by a low mom the Washoe Valley. This valley in turn continues nocivi the Truckee Meadows or the Reno Valley of the Track Rover The mountains to the east of these valleys are of the Chark Great Basin type-short detached harren ranges gung permanent streams, but eroded into numberless waterless d and washes. The Sierra Nevada Range occupies the southem, pl Markleeville sheet, its highest peaks reaching to an atti |