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TABLE No. 6.- Population of principal communities in the Purgatoire watershed
15. Natural resources.-Coal is the principal mineral resource of the basin, and it is mined at various locations west of Trinidad. The potential coal still available in the area has been estimated by the Colorado State Geological Survey to be 24,000,000,000 tons. Other nonmetallic minerals, found to a limited extent in the western portion of the watershed, are fire clay, granite, limestone, sandstone, and graphite. Natural gas is found east of Trinidad near Garcia, Colo. (mile 149), where there is also a natural gasoline recovery plant. Helium gas was developed and refined for commercial purposes on the Model Dome (mile 97) in the north central part of the basin near Thatcher, Colo., for several years. The United States acquired the Model field and refinery in 1937. There is a limited amount of merchantable timber in the mountainous sections of the headwaters.
16. Agriculture.- In the portion of the valley under consideration, the majority of the agricultural improvements are located in the areas where irrigation has been possible. Dry-farming areas represent less than 1 percent of the agricultural developments. The average annual value of the crops in the flood plain is about $48,300. The principal crops in the order of their importance are sugar beets, tame hay, corn, beans, and small grains. A large part of the produce is used locally as winter feed for cattle and sheep. The distribution, yield, and value of the crops are given in table No. 5, appendix C.
17. Irrigated lands.-The irrigated acreage in the basin, most of which is located outside the flood plain, lies mainly in two large areas between Trinidad and the head of the canyon section; namely, the Sunflower Valley area, and the area served by the Model Land & Irrigation Co. The ditches serving these areas are owned by operating stock companies, with water attaching to the stock shares, and most of the shares are owned by the irrigators. The Sunflower Valley area is served by a ditch, water to which is diverted from the river by the Baca Dam (mile 156.4), the enlarged south side (Pulaski) ditch (mile 153.4), and other smaller ditches. The Model Land & Irrigation Co. maintains a ditch and reservoir system which was originally designed to irrigate approximately 20,000 acres of land. The reservoir, with a present capacity of about 15,000 acre-feet, is located on the drainage area of Chicosa Creek which enters Purgatoire River at about mile 140.' Water is diverted into the reservoir by the Model Dam, aflow dam in the river at mile 151.9. Because of other 1 Not printed.
decreed water rights, the diversion of water to the reservoir is made mainly during the winter months and during high flow periods at other seasons of the year. All other irrigated lands in the vicinity are served by direct diversions without the aid of reservoir storage.
18. Irrigation improvements in the basin consist principally of small dams and irrigation headworks, and canals. From the mouth of Long Canyon to mile 122 at the head of the canyon section, 7 low concrete dams have been constructed with heights varying from 3 to 7 feet. Nine Mile and Highland Dams are permanent structures about 5 feet in height in the lower canyon section. There is no storage capacity behind any of the diversion structures, the dams serving merely to raise the water to the elevation of the diversion canal headgates. There is also a number of other irrigation structures consisting largely of small brush and rock diversion dams of a temporary nature, with the necessary ditches leading the water directly to the farm lands. There are records of 22 reservoirs in the basin with an aggregate capacity of 29,000 acre-feet at the time of construction. None of these reservoirs is on the main channel of Purgatoire River. At the present time, several diversion structures are not in operation because of damages sustained during the flood of April 1942, and reconstruction plans are as yet in the preliminary stages. The irrigation ditches with decreed rights drawing water from that part of Purgatoire River in the area under consideration prior to April 1942, with the priorities and amounts of water decreed to each, the length of ditch, and the acreage under each ditch are shown in table No. 3, appendix C.
19. Occupations and industries.--The principal industries of the basin in order of economic importance are mercantile, coal mining, agriculture, and manufacturing. The trend of economic cycles is reflected by the coal-mining activity within the basin. Many of the miners engage in raising livestock and farming during slack periods. Retail business in Trinidad serves the greater part of the population within the basin, and in 1940 the number of these establishments was 211, with a total volume of business amounting to $5,761,000. According to statistics published by the State of Colorado, Las Animas County, in which the greater portion of the Purgatoire River Basin lies, ranked first in the State in 1940 in the value of range cattle and second in coal mining in 1939. The values of these products were $2,000,000 and $2,850,000 respectively. Manufacturing consists largely of the processing of clay products, foundries, stone, and broom works, and the processing of meat and farm and dairy produce. Logging operations are conducted to some extent in the mountains, the products consisting principally of mine props and tics. A small portion of the San Isabel National Forest is in the extreme northwestern part of the basin. The main trunk pipe line of the Colorado Interstate Gas Co. traverses the central part of the basin in a north-south direction, crossing Purgatoire River at mile 97. The line transports natural gas from the Texas gas fields to cities north of the drainage area and through connecting lines serves States adjacent to Colorado.
20. Railroads.- The Atchison, Topeka, & Santa Fe Railway crosses the basin twice, once in an east-west direction at the mouth of the river, and once in a southwest-northeast direction through Trinidad.
* Not printed.
The Colorado & Southern Railway provides rail facilities for north-
21. Highways.---Federal Highway 87, which extends from the Gulf
22. Äirport.-The municipally owned Trinidad airport and landing field is about 12 miles east of the city. There are no commercial air lines operating regular schedules from this field. The United States Army uses the field for routine and emergency facilities.
23. Bridges. There are 10 railroad, four Federal and State Ilighway, six county, and three city bridges crossing Purgatoire River. Several structures also span numerous tributaries. Purgatoire River and its tributaries are not navigable, therefore a War Department permit was not required for the construction of any of the bridges.
24. Precipitation. There were eight operating precipitation stations within the watershed at the beginning of 1942. Southeastern Colorado, in which the Purgatoire River drainage area is located, is a semiarid region, but the orographic effects contiguous and within the basin produce a marked precipitation differential between specific locations. At Las Animas, Colo., which is adjacent to the basin at the river mouth, precipitation records were obtained during 1862. The records for this station are essentially continuous from 1867 to date and constitute the longest record. The maximum yearly precipitation recorded at this station was 21.39 inches in 1923. Trinidad has the maximum recorded annual precipitation, 34.48 inches in 1878. The minimum annual precipitation recorded in the drainage basin was 2.79 inches at Las Ánimas in 1894. The greater portion of the annual precipitation occurs from April to September, inclusive. The average monthly, the percent of average annual, and the average annual precipitation for United States Weather Bureau stations at Las Animas, Trinidad, and North Lake are given in table No. 3. Appendix B.'
25. Snou fall.-Snowfall is general over the Purgatoire River Basin from October to April, and remains on the ground throughout the year at some points in the higher altitudes of the western portion of
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the watershed. The snowfall is heavy in the mountainous headwater region and decreases in amount toward the mouth of the river. Approximately one-half of the average annual precipitation at North Lake occurs as snowfall, and only about 15 percent of the average annual precipitation at Las Animas is in the form of snow. The average annual snowfall varies from about 19 inches at Las Animas, elevation 3,899 feet, to amounts exceeding the approximate average annual of 115 inches recorded at North Lake, elevation 8,800 feet. The one snow course in the drainage basin is located on Whiskey Creek, a tributary of North Fork, in the headwater area. The average snow on the ground, April 1, for the 3-year period of record, was 27.5 inches. The average snow-cover measurements on April 1 for the period of record, 1941 and 1942, are given in table No. 5, appendix B.
26. Meteorological characteristics.- The major topographic features lying between affected areas in New Mexico and southern Colorado and the source region of air masses producing precipitation, indicate that the general effects of these topographical features upon the invading air masses is the critical factor in regulating major precipitation therein. Studies of these features dealing with the general meteorology of the region are discussed in appendix B. Analyses of rainfall records in Colorado and New Mexico indicate thai polymorphous storms occur within the Purgatoire River Basin, and that large magnitude storms usually occur during the period from April through October. The general storm, of which two types occur, and the thunderstorm (local summer storm) produce consummated precipitation.
27. One of the general storms is identified as the stable wave or partly occluded type. These storms cover comparatively large areas, intensities are usually low, and being frontal, show the effects of orographic influence. The storm periods may cover several days.
28. The other general storm may cover large areas, but owing to the nature of the storm type, a front aloft, precipitation should not be influenced by orographic effects. Further, the exact location of the precipitation area is a random affair depending upon the rates of movement and the intensities of the air masses involved. These storms may persist for periods in excess of 24 hours, but are usually of shorter duration. Intensities may be comparatively high at several locations within the general storm area, tending to indicate a series of local thunderstorms.
29. The local summer storm represents a form of penetrative convection due to insolational heating of the lower layers of the atmosphere and orographic influences. These storms are of short duration, intensities are relatively high, and they cover comparatively small areas.
30. Stream-gaging stations.- Run-off records are available at 14 stations with periods of record varying from a few months to 21 years of complete record. Stream-gaging stations within the Purgatoire River Basin are listed in table No. 6, appendix B.' Records were first obtained at Trinidad in 1897. The record at this station is continuous from 1920 to date, but prior to 1922, when an automatic gage was installed, the records are not in sufficient detail to obtain reliable estimates of flood peaks. The maximum observed peak discharge of 64,500 cubic feet per second in Purgatoire River occurred during the
flood of September 15, 1934, at Nine Mile Dam, in the lower reaches of the river. The average annual discharge of Purgatoire River at Trinidad for the period October 1920 to September 1941, inclusive, was 62,522 acre-feet, about 83 percent of which occurred during the months from April through September. In addition to this volume, it has been estimated that an average of approximately 25,000 acre-feet per year has been diverted for irrigation purposes upstream from Trinidad. During the period, the annual run-off has varied from a minimum of 24,730 acre-feet to a maximum of 131,450 acre-feet during the first 9 months of 1941, which exceeded the annual run-off during all other years of record. The monthly, annual, average monthly, and average annual discharges of Purgatoire River for the periods of record at Trinidad, Nine Mile Dam, Highland Dam, and Las Animas are shown in tables Nos. 7, 8, 9, and 10, respectively, appendix B.
31. Run-off.- Purgatoire River is perennial upstream from Trinidad and the greatest part of the volume of run-off occurs during the period when melting of the snow blanket in the mountains begins and continues during the months when thunderstorms occur most frequently,
32. Studies of run-off records of floods resulting from general storms indicate that the shape of the flood wave is extremely sharp for a drainage basin which is narrow compared with the length. It may be deduced that flood-producing precipitation either progresses downstream causing synchronization of large discharges from the tributaries with the peak in the river, or orographic influences control flood-producing intensities within areas having comparable times of concentration, or a combination of both.
33. In mountainous areas, where a greater portion of the soil profiles are shallow and undeveloped, the maximum rum-off conditions may prevail during periods when storms producing maximum intensities do not occur. This apparent paradox is explained by the fact that loss rates prevailing during storms of long duration and of relatively low intensity, may be much smaller, thus producing greater run-off than that produced during high-intensity storms when losses due to infiltration rates are large. This anomaly may result in run-off food waves of comparable peak discharge, but when loss rates prevail, the run-off hydrograph will indicate a larger volume of run-off produced by a smaller amount of precipitation at lower intensities over the drainage area, than for a comparable peak discharge when infiltration rates prevail.
34. Floods of record.--Although continuous stage records of flood run-off are available for a period of only 20 years, historical records indicate the comparative magnitude of the major floods which have occurred in the drainage area since 1866, thus extending the data for major iloods over a period of 76 years. During the period of reliable records, major floods occurred in Purgatoire River on September 30, 1904; July 22, 1925; August 7, 1929; September 15, 1931; and April 23, 1942. Available data indicate that prior to the establislunent of gages, or during periods when the gage readings of the crest stages were not obtained, large magnitude Hoods occurred at Trinidad in November 1866, the summer 1883, and July 1886, and in the lower reaches of the river on Septmeber 16, 1875, and October 19, 1908. The major flood occurrences are discussed in detail and relevant data for these and other floods are given in Appendix B.'