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002~~n.. CHAPTER I--INTRODUCTION ~ 1. GeneralDescription. .. Bonny Dam is a major rolled-earth structure. It is the bulwark of the St. Francis unit, a multipurpose development which is a part of a co- ordinated plan to control and develop the upper Republican River area. Bonny Dam is constructed on the South Fork of the Republican River, in section t5, T.5 S. ~ R. 43 W.. approximately 2 miles west of Hale. Colo., and 29 miles southwest of St. Francis, Kans. Its relationship to 20 other dams planned for construction within the Kansas River basin of the Missouri River Basi.n project is shown i.n figure 1. Bonny Dam was built to provide irrigation and flood control benefits to an area frequented by severe droughts and devastating floods. Other benefits incLude fish and wildlife preservation, abatement of stream polution, stream regulation, and the creation of facilities for recreation. 2. Physical Geo~raphY. - The high plains of northeastern Colorado and northwest- ern Kansas are draine by the South Fork of the Republican River. This river, with headwaters in Lincoln, Kit Carson, and Yuma Counties in eastern Colorado. flows north.. east into Kansas near Hale, continues northeast about 36 miles, and enters Nebraska near Benkleman as shown in figure 2. The drainage area above the dam site is rather flat and sandy. It is largely pasture land covered with native sod and very susceptible to both wind and water erosion where the native vegetation is absent. Below the dam the drainage area is fairly steep in the potential pro3ect area. The upland is divided into long and rather narrow strips by steep eroded valleys which extend back from the river into the tablelands. These drain- age ways cut the terrace lands into rather sma 11 areas. The terrace lands are rolling. with gentle slopes merging into the first bottoms which are nearly level to gently undu- lating. Elevations range from 3614 to 5250 in the river channel. Native vegetation consists primarily of short grasses, with grama and buffalo grass predominating. In some of the bottom lands where drainage is poor, the main vegetation is salt grass, slough grass, sedges, and cattails. Much of the bottom land is now covered with witlow and cottonwood trees. On hills and Slopes where fertility is low, sage brush and yucca are found. " The geology of this area closely resembles that of the Great Plains as a whole. Over a long geologic period, sediment has been deposited to a thickness of 4,000 to 6,000 feet. A loess mantie covers the high land. Underlying the loess is the Ogallala sandstone formation and underneath this is impervious Pierre shale. The valley has been cut rather deeply and later partially filled with sand and alluvium. This alluvium is largely composed of the coarser material that remains from the breaking down of the Ogallala beds. There is some indication that the filling of the valley with coarse mate- rials may be continuing. Deposition of almost all of the material round near the surface has been by wind action. The loess is a well-graded, silty, fine sand. Stream-trans- ported sand and silt formed the Ogallala sandstone bed. Sedimentation in the sandstone has been caused by an infiltration mainly of calcium carbonate. Much of the water from rainfall percolates through the topsoil and the older tertiary beds, and is carried to the stream valley as ground water. The watershed is subject to high winds, rapidly changing temperatures, hail, lack of moisture, occasional ravaging floods, blizzards of severe intensity, hot dry summers, and cold winters. The climate may be classed as semiarid, the average rainfall being about 20 inches. Approximately 85 percent of the annual precipitation occurs during the growing season. April through October. It is not very well distributed, however, and extended dry periods are frequent and of considerable duration. The ex- tremely hot, dry summers are accompanied by low relative humidity and shade tempera- tureS up to 1100 F. Hot winds are not unusual and leave devastating results. The grow- ing season in Cheyenne County, Kans., averages 160 days. with the average last killing frost on May 2 and the first killing frost on October 9. Although the greatest recorded snowfall in a 24-hour period was 12 inches, wind and blizzard conditions increase the winter hazards. An average of [our hailstorms occur each year in Cheyenne County. Crop damage from this source is about once in every 4 or 5 years. .