for a short distance above and below the section, and has a good gravel bed.

THE SUN RIVER.

The mountain catchment area of the Sun River lies at an elevation of from 4,500 to 7,000 feet. It is heavily timbered and has highly inclined slopes well covered with soil. The total drainage area is estimated to be 2,240 square miles. The river has a fall of about 40 feet to the mile, a rocky bed, banks generally high, but in some cases low, with bluffs on at least one side of the river. It frequently flows through narrows, canyons, and deep gorges, and is free from sediment except in flood season. Below the catchment area the river flows through a narrow valley, having an elevation of from 3,300 to 3,800 feet, with broad bench lands on the left bank, extending to the Missouri River on the east and to the Teton River on the north. On the right or south bank the bench lands are more limited in area, but are nevertheless of large extent. A portion of the country is rough and broken, with "flats" here and there..

The regular floods are caused by the melting of the snows in the mountains, which sometimes disappear with great rapidity, due to the "Chinook wind." The snows which fall in April and May are melted by the warm rains before they become consolidated, and thus irregular and overwhelming floods are liable to occur. The water for irrigation is taken out by means of small ditches only. There is one good-sized ditch partially completed, on which no work has been done for the last two years.

The gauging station is about 18 miles above the town of Augusta and 58 miles from Craig, the most convenient railroad station, and it is above all the irrigating ditches. Measurements were begun August 5, 1889, and carried on during September, October, and November. In the following spring a better locality was chosen about 200 yards below, and observations of height and discharge begun on April 1. The area of catchment above the gauging station is about 1,175 square miles.

CACHE LA POUDRE BASIN.

The gauging station on the Cache la Poudre was constructed in the spring of 1884 under the supervision of Mr. E. S. Nettleton, at that time State engineer of Colorado. A record of the amount of water passing this station has been kept, usually from March to October of each year, since that time, giving the longest record yet obtained of the annual fluctuations of any stream in this part of the arid region. During 1889 this Survey cooperated with the State engineer in continuing the work at this important point to preserve an unbroken record of the summer discharge. The station is about a half mile above the mouth of the canyon, and 12 miles above Fort Collins. A description of the station, the equipment and self-registering height gauge or milometer, together with results, may be found in the second, third, and fourth biennial reports of the State Engineer to the governor of Colorado.

The results obtained here are of more than local interest; they may be taken as in a certain degree representative of the condition of other streams, not only in Colorado, but over a large portion of the arid region, in the gradual decrease of discharge during the last few years, culminating in the unusually dry seasons of 1888 and 1889, when there was great loss and even suffering in many settlements in Colorado, New Mexico, and Utah, due to shortage of water and failure of crops.

The mean discharge of the several years, as obtained by the State engineer and by this Survey, is given in the tabulations further on.

In the following table of the average daily flow by months the great decrease from year to year can be seen by inspection.

Mean discharge of Cache la Poudre River in second-feet.

THE ARKANSAS BASIN.

The Arkansas is, in many respects, a typical river of the arid region, in both its topography and regimen. Geographically, it is intermediate between the large rivers of Montana, with their superabundant water supply and regular spring floods, and those of Arizona, with usually small flow and spasmodic, disastrous deluges in the late fall or winter months. It rises in the snow-mantled mountain slopes of central Colorado in an area varying in altitude from about 5,000 feet at the eastern base to 10,000 on the west, and dotted over with peaks rising to 14,000 feet above sea level.

At the higher altitudes (above 10,000 feet) most of the precipitation is in the form of snow, which accumulates from October to April. The melting of this snow is the source of water for the river during the irrigating months of spring and early summer. The snow falls frequently, and the winds blow it into the gulches and ravines until these are often filled to a depth of 50 or more feet. Were the snow uniformly spread on the ground a large portion would evaporate without melting or would melt much more rapidly in the spring, causing higher and shorter floods than is now the case. Though as a whole this portion of the arid belt is denied a rainfall sufficient for the needs of agriculture, this condition is somewhat ameliorated by the fact that the mountain areas receive more than their share of moisture in the form of snow. This, being stored up during the time of the year when not needed and loosed when the requirements of irrigation demand, is borne downward by the rivers to be delivered on the land by the irrigators in such time, quantity, and place as are needed. At present there is a time of scarcity in early spring and in July and early August, and it is the inability to get water at this time that limits agricultural development. To supplement the action of nature, storage reservoirs are built to furnish water in these months.

Above Canyon City, Colorado, the Arkansas River has, in the main, the character of a mountain torrent, descending from an altitude of 10,000 feet near Leadville to 5,300 feet near Canyon City, a distance of 120 miles. Below that point it be

gins to traverse the Great Plains, and assumes a different type; its gradient is diminished, its breadth increases, and, owing to its reduced velocity, it assumes a sinuous course. With this lessened velocity it is unable at ordinary stages to carry the load of detritus collected in the more rapid portion above, and this is deposited, forming low, sandy banks and bars, blocking the course and causing the stream to shift its bed. But at high stages this material is again caught up, the banks are eaten away, and the loops are sometimes cut. Very considerable changes of channel are thus produced by a single flood. Owing to this instability of the bed and banks of this portion of the river, dams and other headworks for irrigating canals are constructed and maintained with difficulty.

On the headwaters of the river are eight gauging stations, located chiefly at points where the results will be of greatest value in any discussion of storage problems. At these altitudes the climate is too cold for crops to mature, except hay, of which but one cutting is made in a season.

The first and highest of these gauging stations is on the East Fork of the river, about 3 miles north of Leadville, at the outlet of a reservoir site examined by this Survey in 1889. The next is on the Tennessee Fork, about 5 miles from Leadville and mile from its junction with the East Branch. The third station is on Lake Fork, about 6 miles from Leadville.

About 10 miles below the union of the forks there is, on the main river, a fourth station, 11⁄2 miles below Hayden, a stopping place on the Denver and Rio Grande Railroad. At this point are to be the headworks of the diversion canal, outlined in the report of Mr. S. H. Bodfish, Division Engineer, to take the water from the river around the base of the hills into the Twin Lakes Reservoir site. Gaugings are also carried on below the outlet of these lakes to obtain the present discharge.

The sixth station is on Clear Creek, the next tributary coming in from the west, about 4 miles below Twin Lake Creek. The measurements are made a few hundred yards above the place where it empties into the Arkansas. The two remaining gauging stations are on Cottonwood Creek, a stream which enters also from the west about 15 miles below Clear Creek.

The measurements are made on the Middle and South Forks just above their junction and about 7 miles from the town of Buena Vista. These branches are typical mountain streams— their fall is rapid and the channels are blocked by bowlders and fallen timber, rendering it a matter of difficulty to find a place suitable for gauging.

Measurements were begun in the spring of 1890, as soon as the roads were opened, and were carried on continuously. The results obtained are shown in the following table:

Mean discharges of tributaries of Arkansas River above Canyon City, Colo., 1890, in second feet.

The principal station on the Arkansas River is near Canyon City. This was located in the spring of 1888 by the State Engineer of Colorado at a foot bridge crossing the river near the Hot Springs Hotel, about 2 miles above town. Previously gaugings had been made farther down the river, at Pueblo in 1885 and 1886, and at a point 9 miles above Pueblo in 1887, but both of these places had been abandoned as unsuitable from the frequent change of cross-section. In 1889 the Canyon City station was reestablished by this Survey and work carried on continuously from that time. The discharge at this point gives the flow of the river little diminished by diversions for irrigation, as at this locality the stream makes its exit from the mountains. Above this point there is comparatively little irrigation and there are small possibilities of future development of it, in the few small patches of land adjacent to the river. The channel here is straight for several hundred yards both above and below, the current is neither too swift

"See Fourth Biennial Report of the State Engineer to the Governor of Colorado, p. 58.