With the exception of the Pacific coast north of San Francisco and some of the mountainous regions, the states east of the tier of states along the 100th meridian receive more precipitation than those to the west. The riparian doctrine applies, except where recently modified by statute, in all of the states east of the tier of states bisected by the 100th meridian. Figure 3 is a map showing the relationship of average annual precipitation to the water rights doctrines of the conterminous 48 states. The average annual precipitation of each of the 11 western states, in inches, is presented in table 2. The weighted average annual precipitation throughout the 11 western states is about 17 inches. A generalized discussion of the characteristics and distribution of precipitation of the eleven western states is presented in appendix C. Groundwater Resources Groundwater occurs in numerous environments in the western United States. These environments include the mountainous areas, the vast alluvial basins, the volcanic rocks of the Pacific Northwest, the consolidated sandstone aquifers of the Colorado Plateau and Wyoming Basin, the alluvium of the High Plains, the consolidated sandstone aquifers along the eastern portion of the study area, and the glacial deposits of northeastern Montana. 4/ The mountains of the western United States, primarily National Forest lands, receive the major portion of the precipitation and yield the major quantities of runoff. These mountains, mainly composed of hard, dense rock, yield only small quantities of groundwater to wells. In addition to the thin soils that cover portions of the mountain slopes and the scattered fractured zones, groundwater also occurs in the alluvial deposits of the intermountain valleys. Of these alluvial deposits, the glacial outwash deposits are probably the most productive. The total groundwater storage capacity of these intermountain alluvial deposits is relatively small. Between and adjacent to the mountain ranges vast deposits of silt, clays, sands and gravels, fill alluvial basins in the Great Basin, Lower Colorado, California and Rio Grande Regions and in the Puget-Willamette Trough of the Columbia-North Pacific region. For the most part these deposits are saturated by groundwater which is recharged from the adjacent mountain areas. In most areas there are sufficient sand and gravel lenses and beds to provide good to excellent aquifers with vast storage 4/ Id. at 28-55. Arizona: Harshbarger, J.W., 1966, Arizona Water: U.S. Geological California: California State Water Resources Board, 1951, Water Idaho: Water Resources Research Institute, University of Idaho, 1968, Preliminary Inventory of the Water Resources of Idaho: Idaho Water Resources Board, Planning Report No. 1, 1968. New Mexico: New Mexico State Engineer Office, 1967, Water Resources of New Mexico: New Mexico State Planning Office, Santa Fe, New Mexico. Washington: United States Dept. of Agriculture, 1941, Climate and Man, Year Book of Agriculture: U.S. Dept. of Agriculture. Colorado, Montana, Nevada, Oregon, Utah and Wyoming: McGuinness, 1963, The Role of Ground Water in the National Water Situation; U.S. Geological Survey Water Supply Paper 1800. a/ Derived by Resource Contractor. space for water. In many locations the sand and gravel is capable of yielding hundreds or even several thousands of gallons of water per minute to individual wells. Public lands overlie all or portions of many of the alluvial basins. A large area of the Columbia-North Pacific Region is primarily formed by extrusive volcanic rocks interbedded with and overlain by alluvium and lake sediments. Large quantities of groundwater are present in the volcanic rocks. Recharge to the groundwater supply in this region comes, to some extent, from local precipitation. However, the main source of recharge originates as runoff from predominantly National Forest lands which surround much of the area. The lava flows are very thick in many areas and contain numerous permeable zones, and thus have a high overall permeability. Coupled with a sufficient source of recharge and numerous permeable zones these aquifers have a large storage capacity and are highly productive. In addition to the volcanic aquifers, groundwater is also present, in many areas, in the unconsolidated deposits covering and interbedded with the lavas or filling stream valleys. These aquifers are usually not as productive as the lava aquifers. The best unconsolidated deposits are glacial outwash along the streams. Public lands overlie a large portion of the groundwater basins of Oregon and Idaho. The Colorado Plateau and Wyoming Basin area is an extensive area including northern Arizona, northwestern New Mexico, western Colorado, eastern Utah and south central Wyoming. The aquifers of this area are composed primarily of interbedded sandstone and shale, which have a generally low permeability and receive limited quantities of recharge. Thus, these aquifers are not highly productive, with yields of only a few tens of gallons per minute in most places. The storage capacity is also low. Water bearing alluvial deposits are minor in occurence and are scattered throughout the area. The largest are at the foot of the mountains in the northern part of the area. These alluvial deposits are fairly productive aquifers although development has been limited. Typical of the 11 western states, much of the land in the Colorado Plateau and Wyoming Basin area is public land. These are predominantly Bureau of Land Management lands. Except |