1760 REPORT OF THE CHIEF OF ENGINEERS. 



All the streams are of course higher, while in the storm area, than before and after. 

 This was noticed in various rivers, particularly at points of the Chama, the Navajo, 

 and the Blanco, revisited in August, originally passed a month earlier. 



Ascending from the lower valleys to the summits of the mountains and points occu- 

 pied for mining purposes, tin' seasons, with reference to warmth, may he said to con- 

 sist of hut winter and summer, with the latter scarcely a fourth of the former. In 

 many places snow begins falling in October, soon blocking the trails and passage-ways, 

 and continues until late in the spring, the greatest snowfall generally occurring, it is 

 affirmed, in March. 



The deep snows of the long seasons render work impracticable, and often access, 

 save on snowshoes, impossible, until late in June. In fact, July. August. Septem- 

 ber, and October in part, can alone he counted upon as the working-season above 

 ground in a mining-camp. All such are at a great altitude, probably in the main 

 above 11,000 feet, frequently up to and over 13,000. Here it rarely rains; save for 

 scarcely over a month the storms are all of snow, which accumulate to a great depth. 



The precipitation at Denver (elevation 5,200 feet), situated at the base of the mount- 

 ains, was, for the year ending June 30, 1875, 15 inches ; during that closing on June 

 30, 1876, 24 inches; and the average annual rainfall for 5 years, from 1872 to 1876, in- 

 clusive, but 16.1 inches. 



At the Summit mining district, southwest of Del Norte, in the San Juan, at an alti- 

 tude of 11,300 feet, where an accurate meteorological record has been kept for three 

 years, and which was kindly furnished me by Prof. Charles E. Robins, of the Little Annie 

 Mining Company, the annual precipitation of snow was nearly 290 inches, aggregat- 

 ing 24 feet for three years in succession, July being the only month without snowfall. 



The immense precipitation at the summit and like situations along the topmost ridges 

 of the continental divide, where lie chiefly the most valuable mining-camps, and the 

 resultant severity of the long winters, will at once be understood. 



The average annual temperature at the summit for the same period, from 7 a. m. to 

 9 p. m., was 32°, and during eight and one-third months, save on snowshoes, nothing 

 could be done in the way of outdoor employment. 



While this low temperature is the average between the hours named, the heat of the 

 noonday sun is intense, being augmented by tin 1 reflection of the snow and other 

 causes. There exists, in short, a tropical heat of brief duration, preceded and fol- 

 lowed by an arctic cold. 



The rarity of the air and consequent difficulty of physical exertion, the severity 

 of the season, the long intervals of isolation from civilization, and the cessation of 

 mail facilities, exert deleterious effects upon all subjected thereto. Consequently, dur- 

 ing most of the year, mining-camps are found deserted, the population almost wholly 

 departing for lower localities with the advent of the wintry season, returning late in 

 the spring. Illustrative thereof may be mentioned the population of the summit^ 

 which, aggregating 125 in June last, was during the previous winter but 12. 



The comparatively small rainfall of Denver, Colorado Springs, and similar locali- 

 ties of the dry region along the high table-lands and bases of the mountains, may be 

 better comprehended by a glance at the annual rainfall of the following interior and 

 sea-coast points of the United States, obtained from the records of observers of the 

 Signal Office, for the year ending June 30, 1875 : 



Inches. 



Denver 15 



Colorado Springs 11 



Philadelphia . 39 



Washington 31 



Saiut Louis 44 



Boston . 47 



New York 36 



Baltimore 36 



Charleston 69 



Savannah 62 



Jacksonville 58 



New Orleans 75 



High-water season. — With the approach of summer heat the effect upon the great 

 accumulation of snow is apparent, producing the "June rise" of the rivers flowing 

 from the range, wholly a seasonal variation. At this time all streams are at their 

 highest period of the year. Dry arroyas and clayey banks are filled with rushing 

 waters, small brooks are transformed into rivers, and the larger streams, the Rio 

 Grande in particular, become formidable barriers. 



Besides the annual rise in the streams of mountain origin, there is during the 

 season of high water a distinct variation of daily occurrence. This diurnal rise and 

 fall is not abnormal in its action, the period of lowest ebb being about 10 o'clock a. in., 

 due to the great increment of heat accompanying tin; noonday sun. 



