USGS Publications Warehouse

Abstract

The Snake River basin, in southern Idaho, upstream from the mouth of the Powder River in Oregon, includes more than 50 percent of the land area and 65 percent of the total population of the State. More than 2.5 million acres of land is irrigated ; irrigation agriculture and industry allied with agriculture are the basis of the economy of the basin. Most of the easily developed sources of surface water are fully utilized, and few storage sites remain where water could be made available to irrigate lands under present economic conditions. Because surface-water supplies have be come more difficult to obtain, use of ground water has increased greatly. At the present time (1959), about 600,000 acres of land is irrigated with ground water. Ground-water development has been concentrated in areas where large amounts of water are available beneath or adjacent to tracts of arable land and where the depth to water is not excessive under the current economy. Under these criteria, many of the most favorable areas already have been developed; however, tremendous volumes of water are still available for development. In some places, water occurs at depths considered near or beyond the limit for economic recovery, whereas in some other places, water is reasonably close to the surface but no arable land is available in the vicinity. In other parts of the basin large tracts of arable land are without available water supply. Thus the chief tasks in development of the ground-water resources include not only locating and evaluating ground-water supplies but also the planning necessary to bring the water to the land. Irrigation began in the 1860's ; at the present time more than 10 million acre feet of surface water, some of which is recirculated water, is diverted annually for irrigation of more than 2.5 million acres. Diversion of this large quantity of water has had a marked effect on the ground-water regimen. In some areas, the water table has risen more than 100 feet and the discharge of some springs has more than doubled. Large-scale development of ground water began after World War II, and it is estimated that in 1959 about 1,500,000 acre-feet of ground water was pumped for irrigation of the 600,000 acres irrigated wholly with ground water in addition to a substantial amount of ground water pumped to supplement surface-water supplies. Ground water is also the principal source of supply for municipal, industrial, and domestic use. The water regimen in the Snake River basin is greatly influenced by the geology. The rocks forming the mountains are largely consolidated rocks of low permeability; however, a fairly deep and porous subsoil has formed on them by decay and disintegration of the parent rock. Broad intermontane valleys and basins are partly filled with alluvial sand and gravel. The subsoil and alluvial materials are utilized very little as a source of water supply but are important as seasonal ground-water reservoirs because they store water during periods of high rainfall and snowmelt. Discharge from these reservoirs maintains stream flow during periods of surface runoff. Because these aquifers are fairly thin, they drain rapidly and are considerably depleted at the end of each dry cycle. The plain and plateau areas and tributary valleys, on the other hand, are underlain chiefly by rocks of high permeability and porosity. These rocks, mostly basaltic lava flows and alluvial materials, constitute a reservoir which fluctuates only slightly from season to season. Large amounts' of water are withdrawn from them for irrigation and other uses, and discharge from the Snake Plain aquifer is an important part of the total flow of the Snake River downstream from Hagerman Valley. The ultimate source of ground water in the basin is precipitation on the basin. In the mountainous areas, aquifers mostly are recharged directly by precipitation. On the other hand, in the plains, lowlands, and valleys which contain the principal aquifers, far more water is consumed than falls on the area and direct precipitation is a minor part of the recharge. Most recharge to the aquifers is by percolation from streams, canals, and irrigated lands. For convenience in describing the ground water in various places, the Snake River basin was divided into ground-water subareas, 19 of which are described. The Boise-Payette subarea includes most of the western end of the Snake River Plain in Idaho. About 350,000 acres is irrigated with water diverted from the Boise River, and 110,000 acres is irrigated with water diverted from the Payette River below Black Canyon Dam. Irrigation has raised the water table in the developed areas so that the alluvium and terrace gravels are the chief source of unconfined ground water. Other important aquifers include the Snake River basalt and sand strata in the Idaho formation. Water in the Idaho formation generally is under artesian pressure, and in some areas flows above the land surface. Wells in the alluvium and in the basalt yield as much as 3,000 gpm (gallons per minute). Some wells in the Idaho formation yield 1,000 to 2,000 gpm. In much of the area, the water table is near the land surface and many wells are less than 100 feet deep. Wells in the Idaho formation range from a few hundred to more than 1,000 feet in depth. It is estimated that about 500,000 acre-feet of ground water is potentially available for use each year and that 200,000 to 250,000 acre-feet of water could be withdrawn each year with a resultant benefit to waterlogged land. The Mountain Home plateau includes the rolling upland surrounding Mountain Home and extends northwestward nearly to Boise. It is underlain chiefly by the Idaho formation and Snake River basalt. There is little surface-water irrigation on the plateau and ground-water use is not large. Recharge to aquifers in the basalt and the Idaho formation is from precipitation on the area and underflow from the mountains along the north flank of the plateau. In the irrigated area around Mountain Home, the water table is near the surface but elsewhere it generally is several hundred feet below the surface. About 400,000 acres of arable land on the plateau are suitable for irrigation. Although the presently available supply of ground water probably is not large, irrigation of substantial acreages of land with surface water diverted to the area would change the ground-water regimen. The water table would rise considerably, and lithologic units now dry would become saturated. Some of these would be excellent aquifers. It is estimated that 30 to 40 percent of the water diverted to the area would become ground-water recharge, some of which would be avail able to supplement the surface-water supplies. The Snake River valley includes the narrow lowland area along the Snake River from Homedale to King Hill and the lower drainage basins of tributary streams within the reach. The alluvium along the principal streams contains unconfined ground water and yields small to moderate supplies to wells, especially along the Snake and Bruneau Rivers. The Idaho formation, intercalated and underlying basalt, and silicic volcanic rocks yield small to large supplies of warm artesian water to wells. Yields of wells are as much as several thousand gallons per minute; many of the larger producing wells are -re than 1,000 feet deep. Recharge to the artesian aquifers is largely from precipitation on the upland area south of the valley. Natural discharge is into the Snake and Bruneau Rivers through springs and seeps. Much of the artesian water is marginal in quality for irrigation use because of high percent sodium and high sodium-adsorption-ratio. Some of the water is unsuitable for domestic use because of excessive fluoride content. The Owyhee upland includes the Owyhee Mountains and adjacent plateau area in Owyhee County. The area is thinly populated, and the land is used chiefly for grazing. The chief aquifers probably are the silicic volcanic rocks that crop out over much of the area; however, few wells have been drilled and little is known regarding their characteristics. Recharge is from precipitation on the area. The depth to water probably is great over much of the area. At a few places the water table is shallow and some springs are utilized for watering stock. The Sailor Creek subarea lies east of the Owyhee upland, between the Bruneau River and Salmon Falls Creek. All streams are intermittent or ephemeral. The area is thinly populated, and there are less than a dozen wells iv an area of more than 1,000 square miles. Most of these yield water for stock and domes tic use from sand strata in the Idaho formation. A few wells yield water from silicic volcanic rocks. The water table is from a few to more than 500 feet below the surface. The Twin Falls South Side Project subarea includes a strip about 40 miles long and 12 miles wide adjacent to the south side of the Snake River east of Salmon Fails Creek. About 200,000 acres is irrigated in the district. Irrigation began in 1905: because the underlying basalt and interbedded sedimentary deposits were not very permeable, the water table rose rapidly. It is estimated that originally the depth to water averaged about 250 feet. Waterlogged areas appeared by 1912, and many drains, tunnels, and drainage wells were constructed to alleviate seeped conditions. Little ground water is used for irrigation, but many wells are used on farms and to supply towns and industries. The Salmon Fails subarea lies south of the Twin Fails South Sid? Project area and east of Sahnon Falls Creek. About 30,000 acres of land receive a generally inadequate supply from the Salmon River Canal Company system. Water is diverted from a reservoir on Salmon Fails Creek. Ground water supplements surface-water supplies to a small degree. About 80,000 acres of land is suitable for irrigation. Silicic volcanic rocks underlie the entire area and are overlain by sedimentary rocks and basalt. Each of these units is an aquifer at some places, but basalt is the most important aquifer. In general, the basalt in this area is much less permeable than the basalt north of the Snake River--and some wells yield only small to moderate quantities of water. Several wells have been drilled in ore permeable basalt and yield as much as 2,700 gpm. Recharge to the aquifers is estimated to be about 100,000 acre-feet a year. The Dry Creek district, part of the South Side subarea between the Snake River and the Rock Creek Hills, contains about 40,000 acres of irrigable land. The northern part is supplied with water from the Snake River. Most of the remainder of the area receives water from Dry and Rock Creeks and ground water pumped from alluvium, Snake River basalt, and silicic volcanic rocks. The silicic rocks yield small to moderate amounts of artesian water to wells in the southern part. Wells in alluvium, which blankets the silicic volcanic rocks and basalt, yield small to moderate amounts of unconfined water in the central part. The basalt yields small to large amounts of unconfined water in the northern part. The depth to water ranges from 35 to 40 feet near Murtaugh to more than 450 feet on the slopes of volcanic buttes. The artesian aquifer is recharged by underflow of precipitation on the Rock Creek Hills south of the area. The basalt and alluvial aquifers are recharged by infiltration of surface water used for irrigation, leakage from the underlying artesian aquifer, and precipitation. The artesian aquifer is extensively developed, and well interference and declining water levels are common. The unconfined water, especially in the basalt, could be more fully developed, but much of the area underlain by basalt has an adequate surface-water supply. The lower Goose Creek valley in the South Side subarea include a roughly triangular area south of the Snake River between the Rock Creek ]Tills and the Albion Range. The valley floor is underlain by silicic volcanic rocks, Snake River basalt, and alluvium. In the southern part, all three formations yield small to large quantities of water to irrigation wells. In the northern part, basalt is the principal aquifer and yields large quantities of water to wells ; the silicic volcanic rocks there are far below the land surface and have been little explored. The alluvium in the extreme northern part contains abundant water at shallow depth, but it has not been extensively develop because of an adequate surface-water supply. Depth to water ranges from a few to more than 450 feet. Ground water withdrawals for irrigation are estimated to have been from 90,000 to 100,000 acre-feet in 1958. Water from Goose Creek is stored in the Goose Creek Reservoir and used on some of the land in the southern part. In the northern part, surface water from the Snake River is used in the Burley Irrigation District. The surface-water developments help replenish the ground-water reservoir. Underflow from precipitation on the adjacent mountains and precipitation on the area are additional sources of recharge. The Raft River basin in the central part of the South Side subarea is the largest basin tributary to the Snake River from the south side. The Raft River rises in Utah and flows northward in a broad alluvial valley to the Snake River. In Idaho, the valley is bounded by the Malta Range on the west and the Black Pine and Sublett Ranges on the east. The mountains are composed of silicic volcanic, consolidated sedimentary, and intrusive igneous rocks. The alluvium in the lowland is underlain by fine-grained sediments which probably are lake beds. Snake River basalt blankets the northern part of the valley. Nearly all the surface water is used for irrigation, and large supplies of ground water have been developed. Nevertheless, much irrigable land is used only for grazing. About 20,000 acres is irrigated with surface water, and more than 2:,000 acres is supplied with ground water. The depth to water ranges from a few feet in the alluvium near the Raft River to more than 250 feet at some places in the basalt at the north end of the basin. Yields of the more than 250 irrigation wells in the valley range from 100 to 3,125 gpm, and wells having capacities ' as much as 900 gpm are widely distributed. Specific capacities range from 5 to 325 gpm. Water-level records indicate that ground-water withdrawals have not greatly lowered the water table. The Rockland Valley-Miehaud Flats district is in the eastern part of the South Side subarea and includes the Rockland Valley and the lowlands between Rock Creek and the Portneuf River south of American Falls Reservoir. It lies east of the Sublett Range and north and west of the Deep Creek Mountains. The Rockland Valley has a gently to steeply sloping terrain underlain by silicic volcanic rocks and alluvium. The Michaud Flats is a gently rolling plain underlain by Snake River basalt, lake beds, and alluvium. Little is known about the hydrology of Rockland Valley, but rough estimates indicate that total annual surface- and ground-water outflow from the valley is about 50,000 acre feet. Of this amount, about 13,000 acre-feet is discharged by Rock Creek, the master stream of the valley. In the Michaud Flats district the depth to water ranges from 10 to 200 feet and averages about 60 feet. Confined and artesian aquifers in sedimentary, pyroclastic, and volcanic rocks yield several hundred to 4,000 gpm and average about 1,000 gpm. About 6,000 acres is estimated to be irrigated with ground water. The Eastern Highland subarea includes the foothills, mountains, and intervening valleys between Pocatello and Rexburg. The area is drained by the Portneuf, Blackfoot, and Snake Rivers and Willow Creek. The foothills and mountains are composed of consolidated sedimentary rocks with an apron of silicic volcanic rock on the slopes facing the Snake River Plain. Snake River basalt crops out over a large area in the valleys and intermontane basins. Alluvium in the lower Portneuf River, Marsh Creek, and upper Snake River valleys yields small to large quantities of water to wells. Basalt in the upper Portneuf River valley yields large quantities of water. Small to moderate yields are obtained from the silicic volcanic rocks. The ground-water resources have not been extensively developed, although at Pocatello large quantities are withdrawn for municipal and other uses. Recharge to aquifers from precipitation on the northwest-facing slopes adjacent to the Snake River Plain is estimated to be 40,000 to 75,000 acre-feet yearly. Camas Prairie extends westward from the northwest margin of the Snake River Plain and is the westernmost of the major drainage basins tributary to the plain along its northern margin. The drainage basin, which is about 40 miles along and 24 miles wide, is a structural depression partly filled with stream and lake deposits. Snake River basalt crops out in the eastern part of the basin. Shallow alluvial deposits contain water under water-table conditions. Deeper sand strata also contain water, and throughout a considerable area in the central part of the basin the water in these strata is confined under sufficient pressure to flow from wells. Yields of the larger production wells are as much as 1,200 gpm. The Snake River basalt yields 1,000 to 1,300 gpm to a few wells in the south-central and southeastern parts of the basin. Ground- water underflow from the prairie is estimated to be about 20,000 acre feet per year. Big Wood River drains an area on the south side of the mountains of central Idaho. The mountainous parts of the drainage basin are underlain by consolidated sedimentary and intrusive igneous rocks of low permeability. Down stream, in the vicinity of Ketchum and Halley, the river valley is about ? to 1 mile wide and is partly filled with fluvioglacial outwash. South of Bellevue, the valley broadens into a roughly triangular alluviated basin terminated on the south by the Picabo Hills. Both surface- and ground-water outflow from he valley is through gaps on the western flank (Big Wood River valley) and the eastern flank (Silver Creek valley) of these hills. The drainage, area of the basin above Magic Reservoir is about 825 square miles, and average surface-water inflow into the lowland south of Hailey is estimated to be about 380,000 acre-feet per year. Ground water is obtained chiefly from sand and gravel strata in the alluvium at depths of 10 to 70 feet. In the southern part of the basin, water under artesian pressure is obtained at depths of 125 to 150 feet. Yields of the better wells range from about 1,000 to more than 3,000 gpm. Ground-water outflow from the basin is estimated to be about 50,000 acre feet per year. Probably 25 to 50 percent of this could be intercepted. Little Wood River drains an area immediately east of the Big Wood River basin. Most of the basin is underlain by silicic volcanic rocks, but some of the higher ridges consist of limestone, quartzite, shale, and similar rocks. The larger valleys are partly filled with alluvium and basalt. In its lower reaches, Little Wood River is above the water table and loses water by percolation from the streambed. Ground water moves downvalley to join the main ground-water body beneath the Snake River Plain. wells in the vicinity of Carey obtain water from sand and gravel, generally at depths less than 150 feet. A few wells east of Carey obtain water from basalt. The altitude of the water table near Carey is about 4,750 feet, but a few miles southeast of Carey it is at an altitude of 4,050 to 4,100 feet, a drop of 650 to 700 feet in a distance of a few miles. The discharge of the Little Wood River at the gaging station northwest of Carey averaged about 97,000 acre-feet per year through 1956. Total water yield of the entire basin is estimated to be about 150,000 acre-feet per year. Fish Creek drains a small area underlain by silicic volcanic and consolidated sedimentary rocks on the south slope of the Pioneer Mountains. Water yield from the Fish Creek basin is estimated to be about 15,000 acre-feet per year, of which roughly 12,000 acre-feet is surface runoff and 3,000 acre-feet is ground water outflow. The Big Lost River basin is the largest basin tributary to the Snake River Plain from which surface discharge never reaches the Snake River. The Big Lost River drains an area of about 1,500 square miles underlain by granite, limestone, quartzite, shale, and silicic volcanic rocks--all are materials of low permeability. The valley of the Big Lost River is about 60 miles long and through much of its length ranges from 3 to 6 miles wide. The main valley and some of the larger tributary valleys are partly filled with alluvial sand, gravel. and clay. These materials are moderately porous and permeable. Above Mackay Reservoir on the Big Lost River, they serve as a large underground reservoir and stabilize inflow to the surface reservoir. Downstream from Mackay Dam, the water table generally is below stream level, and the alluvium serves chiefly as a conduit for transmitting water underground from the basin into the Snake Plain aquifer. Upvalley from Arco, which is near the mouth of the valley, the water table generally is not more than a few tens of feet below the surface. Wells are as much as 250 feet deep and yield as much as 2,700 gpm from the alluvium. Larger yields could be obtained. Because of a progressively increasing amount of underflow downstream, the surface flow at the gaging station at Howell Ranch, about 60 miles upstream from Arco, equals or exceeds that at downstream stations, even though a large amount of water enters the valley below Howell Ranch. The discharge past this station averaged 218,000 acre feet per year through 1956. The total water yield of the basin at the mouth, including both surface and underground flow, is estimated to be at out 330,000 acre-feet per year. The Little Lost River basin is similar to the basin of the Big Lost River. Much of the discharge of tributary valleys reaches the main valley as under ground flow, and the alluvium in the valley serves as an underground reservoir and stabilizes the surface discharge of Little Lost River. Above a partial barrier formed by a constriction in the consolidated rock of the valley walls about 11 miles upvalley from Howe, the water table is near or at the surface along the axis of the valley. Undoubtedly the underflow in the alluvium extending through this partial barrier is considerable. Downstream, the water table is below river level, and additional water is added to the aquifer by losses from the Little Lost River and other streams. Total water yield of the basin is estimated to be about ]50,000 acre-feet per year, of which perhaps 50,000 to 60,000 acre-feet might be consumptively used within the basin. About 60 wells are used for irrigation in the basin. Most are less than ]50 feet deep and yield between 1,000 and 2,500 gpm with small to moderate drawdowns. Birch Creek basin is northeast of the basins of the Big and Little Lost Rivers, and parallels those basins. The broad alluvial fill in the central valley serves both as an underground reservoir and ground-water conduit. Birch Creek is largely spring fed and the flow is remarkably uniform throughout the year and from year to year. The average measured discharge at the gaging station about midway between the head and mouth of the basin is about 80 cfs (cubic feet per second), or 58,000 acre-feet per year. The total water yield of the basin, both surface and ground water, is estimated to be about 80,000 acre-feet per year. The southern part of the Mud Lake basin consists of a broad fiat basin in the north end of the Snake River Plain: it contains several hundred square miles underlain by river alluvium and lake beds. Snake River basalt ex tends north and northeast to the foot of the mountains. Several streams rising in the Beaverhead and Centennial Mountains to the northwest, north, and northeast converge, spokelike, on the Mud Lake basin. None of the drain age channels reach the Snake River. The more important streams are Crooked, Warm Springs, Deep, Medicine .Lodge, Indian, Beaver, and Camas Creeks. All except Camas Creek and its chief tributary, Beaver Creek, lose their entire discharge within a short distance after reaching the edge of the Snake River Plain. These two discharge some water into Mud Lake. about 32 miles south of the base of the mountains. Only three of the streams have been gaged, and, because of underflow past the stations and ungaged inflow below the .stations, these records do not give the water yield of the basins. In addition to surface and under ground inflow from the creeks, mass underflow probably is considerable into the Mud Lake area through moderately permeable conglomerate, sandstone, and volcanic rocks which crop out in the surrounding mountains. Snake River basalt is the chief aquifer; northeast of Mud Lake it is at or near the surface. To the south and west, it underlies lake beds and stream alluvium. A geologic barrier extending west and northwest through Mud Lake holds the water table near the surface northeast of the barrier at an altitude of about 4,800. To the southwest the water table drops several hundred feet within a few miles. Several hundred drilled wells yield as much as ]0,000 gpm each for irrigation. The total avail able water .supply in the Mud Lake area (chiefly ground water but including some surface water) is estimated to be about 500,000 acre-feet. Depletion due to irrigation is estimated to be about 90,000 acre-feet each year, and natural losses by evaporation and use by waste vegetation is estimated to be about 80,000 acre-feet a year: about 300,000 acre-feet leaves the area as underflow to the Snake River Plain. The Teton basin trends northward in Idaho near the Wyoming State line and is tributary to the Snake River Plain at the eastern end of the plain. The valley of Teton River is about 20 miles long and 5 to ]0 miles wide. It is under lain by several hundred feet of alluvial sand and gravel which constitute the chief aquifer, recharge is from direct precipitation and from water lost by streams flowing into the valley from the surrounding mountains. The water table is several hundred feet below the surface beneath the higher alluvial slopes near the foot of the mountain ranges but is near or at the surface near the center of the valley, where a strip of land adjacent to Teton River is waterlogged. Ground water for irrigation can be obtained from wells drilled into the alluvium. Properly constructed wells probably would yield 1,500 to 2,000 gpm with small drawdowns. Pumpage of large amounts of water would reduce total basin out flow but would also lower the water table in marshy areas and might reduce the amount of water lost to waste vegetation. The amount of water leaving the basin as underflow is not known. Through 1956, the discharge of the Teton River near Tetonia averaged 280,900 acre-feet a year. The Snake River Plain east of Bliss extends east and northeast from Bliss and the Hagerman Valley for about 200 miles approximately to Ashton. The plain is underlain chiefly by a thick sequence of numerous lava flows of the Snake River basalt, but some lakes and stream deposits are interlayered between flows. On more than half the plain, precipitation is less than 12 inches per year, al though it is nearly 30 inches on the high northeastern end. Development of the approximately 11 million acres of land presently irrigated on the plain thus is dependent largely on water brought into the area by streams and by under ground flow. More than 7 million acre-feet of surface water is diverted, and about 800,000 acre-feet of ground water is pumped for irrigation of farm lands on the plain. The series of basalt flows and intercalated pyroclastic and sedimentary materials that underlie the Snake River Plain east of Bliss is defined at the Snake Plain aquifer. Although an individual flow may not transmit much water and some of the interbeds are not very permeable, the Snake Plain aquifer is one of the world's outstanding water-bearing formations. A large number of pumping tests and many specific-capacity data indicate that the coefficient of transmissibility of the upper part of the aquifer generally ranges from 1 to 10 mgd (million gallons per day) per foot. As most of the wells only penetrate a part of the aquifer, the coefficient of transmissibility of the entire aquifer is greater. Data from pumping tests and laboratory studies indicate that the average co efficient of storage probably is at least 5 percent and may approach 10 percent. Recharge to the Snake Plain aquifer is from (a) precipitation directly upon the Plain, (b) percolation of irrigation water, (c) seepage from streams entering or crossing the plain, and (d) underflow from tributary basins. The water table beneath the Snake River Plain is in dynamic balance between recharge and discharge. To the present time (1959), the greatest changes in the ground water regimen have been those caused by diversion of surface water to various irrigated tracts on the plain. These diversions have increased undertow in the aquifer by about 60 percent. This increased under flow has been accompanied by a water-table rise, estimated, on the basis of a few early reported water levels, to be about 60 or 70 feet in the western part of the plain between Minidoka and the Hagerman Valley. This rise in the water table suggests that if an amount of water equal to the increase in underflow, or about 1.8 million acre-feet per year, were withdrawn from the aquifier by wells, the water table would decline about 60 to 70 feet. Seasonal changes in water levels are correlated with seasonal changes in re charge, either natural or man-made, and produce an annual cycle. Because several wet or dry years may occur in succession, other cycles cover periods of several to many years. Seasonal cyclic changes in water levels correlate with application of irrigation-water diversions in several areas. The fluctuations at the other places correlate with discharge of streams that terminate at the mar gin of the plain. The Snake Plain aquifer discharges into the Snake River between Milner and King Hill. By determining the gain in discharge of the Snake River between gaging stations at those places and subtracting all other sources of inflow, the discharge of the Snake Plain aquifer is estimated to have been about 6,500 cfs, about 4.7 million acre-feet a year, during the past decade. Discharges of many springs in the reach have been measured occasionally or periodically since about 1902. Total measured and estimated discharge from the Snake Plain aquifer by springs was about 3,850 cfs in 1902 and 5,900 cfs in 1956. The 9 percent difference between the observed discharge of 5,900 cfs and the calculated discharge of about 6,500 cfs probably is due to discharge from seeps and springs which cannot be seen or measured. The discharge before any development or other man-made change is estimated at 4,000 cfs, or 2.9 million acre-feet a year. Recharge from direct precipitation on the Snake River Plain was estimated to be about 500,000 acre-feet a year. About 60 percent of this amount probably is concentrated in about 15 percent of the area that is much higher than the rest and receives correspondingly greater precipitation. Recharge by underflow and by streambed percolation from basins along the north flank of the plain totals about 1 million acre-feet a year. Average annual recharge from under flow, stream losses, and irrigation in other parts of the plain was estimated as follows: Upper Snake River, above Firth, 2.4 million acre-feet; Firth to Black foot, 600,000 acre-feet; Blackfoot to Neeley, 1.5 million acre-feet (discharge from the aquifer into American Falls Reservoir and the Snake River in this reach); Neeley to Milner, 500,000 acre-feet; Milner to Bliss, 1.2 million acre-feet. Hydrologic boundaries of the Snake Plain aquifier are of two types: positive (source, or recharge) and negative (barrier). The chief positive boundaries are the discharge areas in Hagerman Valley and American Falls Reservoir. The Snake River and adjacent irrigated lands upstream from Idaho Falls may form an additional positive boundary or at least a partial boundary. The chief negative boundaries are the margins of the plain. Discharge and recharge data, the water-table map, and aquifer boundaries were used in constructing a flow net. Each flow line on this map re-presents an underflow of 200 cfs. The flow net and the water-table map were used to construct a map showing areal distribution of the coefficient of transmissibility of the aquifer. The coefficient ranged from less than 1 million to about 60 million gpd per foot. The quantitative data derived in the report were used in computing probable effects of withdrawing large quantities of ground water from five places on the Snake River Plain. In the Wendell area, computed theoretical drawdowns along the line of wells would range from 6 to 10 feet at the end of the first season and from 7 to 11 feet at the end of the 50 pumping seasons. In the Shoshone Dietrich and Eden areas, drawdowns would range from 7 to 11 feet at the end of the first season and from 10 to more than 13 feet at the end of 50 seasons. In the Idaho Falls area, drawdowns would range from 3/? to 4/ feet at the end of the first season and from 7 to 8 feet at the end of 50 seasons. In the Roberts Plano area, the drawdowns would range from 3 ? to 4 ? feet at the end of the first season and from 5 to 6 feet at the end of 50 seasons. The drawdowns given above are drawdowns in the aquifer immediately adjacent to the wells. Drawdowns in the wells would be greater by the amount of well loss, which generally averages about 1 foot. The Snake Plain aquifer provides an excellent opportunity for artificial re charging operations. At many places the formation accepts recharge water readily; the high coefficient of transmissibility allows the recharge, water to spread widely; a large storage space is available. However, water in storage in the aquifer is a transient resource; when the water table is raised, the gradient toward the discharge area is increased and the natural discharge eventually will be increased. Thus, some recharge water almost inevitably will escape from the area where it is stored, and the percentage lost is in part related to the length of time between recharging and withdrawal. The high coefficient of transmissibility of the Snake Plain aquifer results in a fairly short time between recharge and increase in the natural discharge and is a factor in the length of time that water can be effectively stored. Chief sources of water for recharging the Snake Plain aquifer are the Snake River and Henrys Fork. With present utilization of surface water, 700,000 acre-feet or more of water might be available in about half the year. Little or no water would be available in most of the other years. On the basis of past records, there would be periods of 8 to 10 years when no water would be avail able for recharging and periods of about equal length when a large, amount would be available each year. Several areas on the plain appear to be particularly favorable for recharging. One area is between the Egin Bench and Roberts. A second is south, west of Idaho Falls, and a third is north and northwest of American Falls Reservoir. Detailed studies and analysis are needed to evaluate the hydrologic effects and possible benefits from recharging in these and other potential recharge areas.