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Soil Survey #foundations for buildings should be designed to compensate for the high shrink-swell potential of the sail. Poor drainage, low soil strength, and high shrink-swell potential should be considered in constructing roads. Drainage commonly should precede any construction. Poor drainage and slow permeability should be considered when designing septic tank absorption fields and sewage lagoons. After drainage is established, lagoons can be installed. This map unitrs in capability subclasses IVw, irrigated, and Vlw, nonirrigated. 13-Big Blue clay loam. This deep, poorly drained soil is on low terraces and valley bottoms and on alluvial valley floors. It formed in fine textured alluvium derived from shale. Slope is 0 to 6 percent. Elevation is 7,000 to 8,000 feet. The average annual precipitation is 16 to 20 inches. The average annual air temperature is 42 to 45 degrees F, and the average frost-tree period is 90 to 110 days. Typically, the upper part of the surface layer is very dark gray clay loam about 4 inches thick and the lower part is very dark gray silty clay about 6 inches thick. The underlying material to a depth of 60 inches or more is gray silty clay. Gleyed colors and mottles are common in the underlying material. In some areas the surface layer oam. eluded in this unit are small areas of Alamosa loam, Arboles silty clay loam, Bayfield silty clay loam, seeped, and Sili clay loam. Permeability of this Big Blue soil is slow. Effective rooting depth is about 18 to 24 inches because of the presence of a high water table. Available water capacity is high. Runoff is slow, and the hazard of erosion is slight. This soil has a fluctuating water table that rises to a depth of about 18 inches during most of the year. The soil is subject to occasional flooding in spring and summer. This unit is used mainly for irrigated pasture and as rangeland. The main concerns of management on irrigated pasture are properly using irrigation water and maintaining the desired grasses. An irrigation method that is suited to this unit is flooding from contour ditches. Special care is needed in applying irrigation water. Short irrigation runs and light, frequent irrigations are needed to reduce seepage into the water table. Drainage ditches may also be needed to control the level of the water table. The native vegetation consists mainly o1 tufted . hairgrass, slender wheatgrass, redtop, sedges, and rushes. Proper grazing use as part of a planned grazing system helps to maintain the quality and quantity of the ~eferred rangeland vegetation. Seeding facilitates vegetation of areas that have been depleted by overgrazing, cultivation, and other disturbances. Fencing improves the distribution of grazing and helps to maintain range condition, This unit is used by wildlife such as ducks, cottontail, coyote, and small birds. The availability of moisture because of the presence of the high water table allows production of wetland plants that provide nesting areas and protective cover for wetland wildlife. Wetland areas are easily created. Management for wildlife should include protecting the unit from overgrazing and wildfire and providing drainage. Occasional flooding, wetness, and high shrink-sv,~.ii potential are the main limitations for homesite and ~.•r,an development. These limitations should be overconi~ ,t buildings or other structures are planned. The hazo,.i of flooding, the high water table, and slow permeabilnr should be considered when designing septic tank absorption fields or sewage lagoons. Drainage ant protection from flooding should precede any construction. Absorption fields may need to be lar~~,,. than normal. This map unit is in. capability subclass Vw, irrigated ( and nonirrigated. May 1994