along the edge of the region where recharge water may originate\ from National Forest lands, much of the limited recharge originates on the overlying Bureau of Land Management and non-federal lands.

The groundwater of the High Plains occurs in the remains of a large alluvial deposit. This deposit, once far more extensive than its present limits, is found only in small areas of eastern New Mexico, Colorado and Wyoming. The uniform nature of the alluvium allows it to be classed as a single stratigraphic unit, the Ogallala formation. The deposits are as much as 500 feet thick.

Groundwater in these deposits of the High Plains is recharged primarily by precipitation. This recharge is generally small except in areas where sand dunes and other loose alluvial material is present. Many of the streams that cross the High Plains have cut to bedrock beneath much of the Ogallala formation and no longer contribute recharge water. However, the larger streams have laid down their own alluvial deposits and contribute water to these sources and in some cases these alluvial deposits are in hydrologic continuity with the adjacent and underlying Ogallala formation.

Except for a few scattered sections of Bureau of Land Management land, the lands of the High Plains in Colorado, Wyoming and New Mexico are non-federal.

In addition to the alluvial deposits of the High Plains, the eastern edge of the study area. including portions of eastern New Mexico, eastern Colorado, eastern Wyoming southeastern and south central Montana, is characterized mainly by plains and pla. teaus underlain by horizontal or gently dipping consolidated sedimentary aquifers. The aquifers in most of this area are limestone and sandstone with low to moderate permeability. The yields are generally low, the water of poor quality, and overall these are some: of the least productive aquifers in the United States.

Alluvial deposits of substantial width, thickness and moderate to high permeability are found only along the major streams in this

area.

Non-federal lands overlie most of this area. However, significant areas of Bureau of Land Management land are present in Montana and New Mexico. Small areas of National Forest lands also occur in this region. Recharge to the consolidated aquifers along the western edge of the region probably originates from mountainous National Forest lands.

In north and northeastern Montana unconsolidated glacial deposits are present overlying consolidated rocks. These deposits consist of fine grained rock debris. Productive beds of sand and gravel are lacking except along the water courses.

Bureau of Land Management lands are the predominant public land in this portion of Montana.

Generalized areas underlain by productive groundwater aquifers in the 11 western states are shown on figure 4, a map which is based on the Ground Water Sheet of the National Atlas.5/ The areas indicated are underlain by one or more aquifers capable of yielding to a well at least 50 gallons per minute of fresh water. These aquifers include both unconsolidated and consolidated rocks. This map is highly generalized and is known not to include all the productive aquifers of the western United States.

A general description of the groundwater regions in the 11 western states is presented in appendix D.

Water Yield Characteristics

As discussed previously, precipitation is the major source of all water on or under the public lands. A particular area may have all the physical features necessary to yield large quantities of water but if precipitation is low the resultant water yield will be low.

Other than precipitation, the major factors affecting water yield are elevation, topography, rock-soil cover, and vegetation. All of these factors are interrelated and to some extent dependent upon one another. Man's activities, of course, have a major influence on water yields.

Elevation of the land and extremes in topography have great influence on water yields, insofar as runoff is concerned. Higher elevations generally receive greater amounts of precipitation and steep mountain slopes result in large quantities of runoff. The topography also affects the rock-soil cover. Generally in steep areas the soil cover is thin or absent and rock outcrop is present.

5/ U. S. Geological Survey, National Atlas, Ground Water, Sheet 126, (1967).

Runoff removes most of the soil material preventing the accumulation of a thick soil cover which could be capable of absorbing some of the runoff.

in the eleven western states the lower, flatter elevations are typically covered by thick deposits of permeable alluvium and soil, it should be noted that the soil portion of the cover may be thin and incompletely developed. Therefore, in such areas where the topography is relatively flat and the surface material is permeable, the water yields of runoff are low. However, in most cases in the west, these flat alluvial areas receive little precipitation.

The water yield characteristics of the public lands are hereafter briefly summarized for each of the 11 western states. A more detailed discussion of the water yield characteristics is presented in appendix E. Figure 5 is a map Figure 5 is a map showing the general water yield characteristics of the 11 western states.

Arizona

Water yields are generally low throughout most of Arizona, Yields from National Forest lands of the Mogullon Rim and Kaibab Plateau areas are relatively low and the water yield characteristics fair. The low precipitation and moderately well and well drained soils throughout much of the National Forest areas prevent large quantities of runoff. The National Forest lands of southeastern Arizona yield oven less water.

National Park and Monument lands have low yields and fair to poor water yield characteristics. Lands administered by the Bureau of Land Management, which generally occupy the low areas of little topographic relief, receive small quantities of precipitation and generally have well drained soils. poor water yield characteristics and little runoff,

California

They have

The National Forest and National Park lands of the Sierra Nevada, Cascade and northern Coast ranges are covered by gen erally moderately well to well drained soils except for large areas of rock outcrop. They are characterized by steep topography, receive relatively large quantities of precipitation, exhibit generally good water yield characteristics which results in moderatley high to high quantities of runoff. The yields from National Forest lands occur in such areas as the lower Coast Ranges of southern California where yields are moderately low because of less precipitation..

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