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under date of April 28, 1936, to the Secretary of War, for transmission to Congress. The report, which was unfavorable to a survey at Federal expense of those portions of the streams in Las Animas County, has not been published and Congress has taken no action thereon.
3. Geographical description.--Purgatoire River, locally known as Picket Wire River, drains an area of 3,390 square miles in Costilla, Las Animas, Otero, and Bent Counties, Colo., and Colfax and Union Counties, N. Mex., located in southeastern Colorado and northeastern New Mexico between longitude 103° and 105o and latitude 37° and 38°. (See map No. 1, appendix A.) The roughly crescent-shaped basin is longitudinally alined in a southwest-northeast direction with the convex portion in the southern part of the drainage area. Along the western boundary, which is formed by Culebra Range of the Rocky Mountains, the basin is about 17 miles in width; in the middle portion, about 36 miles across; and at the eastern extremity, about 10 miles between the watersheds. The upper third or westerly part of the basin is rugged and mountainous. The remainder, which extends easterly from the front range, is foothills and plains.
4. North and Middle Forks, which have their sources in Sangre de Cristo Range or more definitely Culebra Range of the Rocky Moun. tains along the western boundary of the basin, form Purgatoire River at their confluence (river mile 186). From this point, the river traverses the central portion of the drainage basin to its confluence with Arkansas River about 3 miles east of the town of Las Animas, Colo.
5. The conformation of the drainage basin has developed a stream pattern in which the tributary drainage areas are comparatively small. The mouths of most tributaries are located only a few miles downstream from their headwaters. Chacauco Canyon, the largest tributary, drains an area of 386 square miles on the south side of Purgatoire River. From its source at the southern divide, just westerly from Mesa De Maya, Chacauco Canyon flows in a generally northerly direction for about 40 miles to its confluence with Purgatoire River at river mile 66. Numerous tributaries enter the river from the north and south, the more important ones to the north in order downstream are Zarcillo, Burro, and Reilly Canyons; those to the south, Lorencito, Long, Raton Canyons, San Francisco Creek, and Chacauco Canyon. Relevant data for the major tributaries are given in table No. 1.
Table No. 1.– Tributaries of Purgatoire River
area of tributary
total Purgatoire drainage
Drainage area above mouth of tributary
180 179 176 175 167 165 164 160 122 66
62 137 386
9,390 Square miles
1.3 3.7 2. 3 1.1
705 1, 226 2, 216
6. Topography.--The relief in Purgatoire Basin varies over 10,000 feet in elevation from its headwaters to its mouth. On the western boundary along Culebra Range, Purgatory, Culebra, and Trinchera Peaks exceed 14,000 feet in elevation. The northern drainage divide from Trinchera Peak descends about 7,500 feet in 30 miles to the vicinity of Ludlow, Colo., along the front range. From this location the watershed is located in the plains region. The southern watershed is roughly parallel to the Colorado-New Mexico State line from Sangre de Cristo Mountains easterly to Mesa De Maya, a distance of about 80 miles. The general elevations along this part of the divide are 13,000 at the western end, 8,000 feet in the vicinity of the headwaters of Lorencito and Long Canyons, and 9,500 feet on Raton Mesa in the headwaters of Raton Canyon, south of Trinidad, Colo. From the maximum elevation on Raton Mesa, the elevation decreases to 6,500 feet in about 10 miles. The remainder of the southern divide is located in the plains area. The drainage basin west of Trinidad, Colo., is rugged and mountainous. East of Trinidad the plains which have been dissected are characterized by plateaus, mesas, and deep canyons. General topography is indicated on map No. 1, appendix A.
7. From the confluence of North and Middle Forks to Arkansas River, Purgatoire River flows through four distinct sections: (1) the mountainous section upstream from Trinidad (mile 186 to 157); (2) an alluvial valley section about 35 miles long below Trinidad (mile 157 to 122); (3) a rugged canyon section about 105 miles long (mile 122 to 17); (4) a second alluvial valley section about 17 miles long (mile 17 to 0). The valley in the mountainous section upstream from Trinidad is narrow, varying in width from a few hundred feet to about one-fourth mile. Between Trinidad and the canyon section, the valley reaches a maximum width of about three-fourths mile. Throughout most of the canyon section, precipitous walls confine the river. Between the lower end of the canyon and the river mouth, the lower valley section reaches a maximum width of about 1 mile. Topographic details of the Purgatoire River Valley from mile 163 to mile 122 are shown on maps Nos. 2 to 7,' appendix A.
8. Geology.--The geology of the watershed is complex due to rapidly changing topography. In the upper headwater read the streams flow over successively younger strata which dip steeply to the east and crop out in north-south belts. The strata consists of poorly consolidated red sandstones and shales interbedded with thin limestone of the Pennsylvanian, and a series of sandstones and shales interbedded with limestones and coals of the Jurassic and Cretaceous periods. From about longitude 105°, west to the vicinity of Sopris, Colo. (mile 162), the river and its tributaries flow over interbedded sandstones, shales, and coals of early Tertiary period (Raton formation). North of the river and west of the town of Bon Carbo, Colo., near mile 170, these strata are overlain by a massive sandstone (Poison Canyon formation). Several igneous dikes are exposed in this general vicinity. From Sopris to the Arkansas River Valley, the bedrocks consist of sandstones and shales interbedded with limestone and coals of the Permian, Jurassic, and Cretaceous periods. Between Sopris and the vicinity of mile 65, the strata dip west and become successively older towards the mouth of the river. Downstream from i These maps are not printed.
mile 65, the rocks dip slightly to the east so that the river again flows over successively younger formations. Between Trinidad and Alfalfa, the river flows through soft shale plains. From Alfalfa to within 17 miles of the mouth, the stream flows over the hard sandstone and shale. Narrow canyons are characteristic of this section except near Higbee, Colo. (mile 38), where the river has cut entirely through the bard sandstones and into the softer Morrison shale, thus widening the valley in this vicinity. Throughout the lower 17 miles of its course, the river again flows through the soft shale plains.
9. Stream slopes—Channel capacities.- In the headwater reaches upstream from Weston, Colo. (mile 180), gradients are several hundred feet per
mile. Below that point to the vicinity of Long Canyon (mile 164), slopes are in excess of 50 feet per mile. From Long Canyon to Alfalfa, the stream bed decreases in elevation at rates of between 50 and 20 feet per mile. In the lower reaches, gradients average about 8.5 feet per mile. Channel capacities do not increase downstream as the stream meanders in the flood plain, except through the canyon section, producing variable bank heights and channel widths. River slopes, channel capacities, and other pertinent data are given in table No. 2. A profile of the river is shown on map No. 8,' appendix A.
TABLE No. 2.-Purgatoire River--River slopes and minimum channel capacities
10. Soils.--Four soils groups found in the Purgatoire River drainage area vary greatly in character. In the upper or western part of the area above Stonewall, the Rough Stony, and Alpine Meadow group are mostly of a complex of barren rough stony land. In the meadows the soils are usually shallow and stony. From Stonewall downstream to the front range below Trinidad, Lithosols cover most of the area. The dominant types in this group are Underwood-Babb and McCammon-Deschutes. These soils are complex, as they are developed on rough terrain from a variety of parent materials. They are mostly shallow, stony, and lacking in very definite profile development and subject to erosion. The Brown soils are found in the plains area. These soils are mostly silt loams, though there are also some 1 This map is not printed.
sandy soils. Alluvial soils are found along the stream valley and cover only a small portion of the area.
11. Vegetal cover. --Vegetative types in general are associated geosandy graphically with the soils groups. In the headwater areas above
timber line, the steep mountain slopes are devoid of cover. In the the timber areas, conifers, aspen, and brush with associated grasses are
found. In the woodland zone upstream from the frontal range, els de pinon, juniper, scrub oaks, and sagebrush with scattered growths of
bunch grass predominate. In the plains regions, short grasses, sche gramma, and buffalo grass are the dominant species.
12. Climate.-Climatic conditions vary considerably within the TOID
Purgatoire River Basin, principally because of the great differences in Supe
topography. Owing to this variation and to the insufficient number
of weather stations within the basin, any attempt to present average age se
values for climatic factors of the basin would be misleading. Consequently, mean values were computed for individual weather stations to indicate conditions in the immediate vicinity of these stations. Generally, the summers are warm and the winters are moderate with
occasional cold waves of short duration. The average monthly and perder
annual temperatures, number of years of record, and the maximum and minimum recorded temperatures at the typical United States Weather Bureau stations, are given in table No. 3. The average annual humidity at Las Animas, contiguous to the drainage area at the mouth of the river, varies from 78 percent for the morning observation to 40 percent for the afternoon observation. The average annual precipitation varies from 12.17 inches at Las Animas to 22.10 inches at North Lake.
Table No. 3.- Average monthly and annual temperatures and maximum and
minimum recorded temperatures
13. Maps.--The watershed is almost entirely covered by the quadrangle maps of the United States Geological Survey, Department of the Interior, which are listed in table No. 4.
TABLE No. 4.—United States Geological Survey quadrangles covering Purgatoire
The drainage basin and other pertinent data are shown on map No. 1, appendix A.
14. Population. The total population of the basin is estimated to be about 26,900 and is largely concentrated in Trinidad and vicinity. The greater portion of the watershed is very sparsely settled. Trinidad, with 13,223 inhabitants, is the largest town in the basin. Las Animas, on the watershed divide near Arkansas River, has a population of 3,232. A comparison of the population figures from the United States census for 1920, 1930, and 1940 indicates that for the entire watershed there was an 8 percent decrease from 1920 to 1930 and a 9 percent decrease from 1930 to 1940. The population of Trinidad has increased in the 2 decades from 10,906 in 1920 to 13,223 in 1940, or an increase of 21 percent. Pertinant data on the trend of population for the portion of the counties within the basin are shown in table No. 5.
TABLE No. 5.- Trend of population in Purgatoire River Basin
All communities wholly within the basin having 300 inhabitants or more are in Las Animas County. The 1940 populations of these communities are shown in table No. 6.