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Anticipated irrigation on sandy soils has prompted evaluation of ground-watersupply potential in the Little Falls area. Geologic conditions cause ground-water availability to vary widely in the area. The largest and most readily available g~oundwater source is the glacial outwash sand and gravel from which the soils were derived. Test augering shows that the saturated surficial outwash is as much as 50-100 feet thick in the area where the outwash fills a probable former meltwater channel and that it is also this thick in smaller areas elsewhere. Transmissivity of the thicker parts of the aquifer approaches or exceeds 100,000 gallons per day per foot, and probable well yields should exceed 1,000 gallons per minute. In about two-thirds of the study area, a saturated thickness of less than 40 feet generally limits well yields to less than 300 gallons per minute. Recharge to the surficial aquifer is obtained primarily from precipitation. Most discharge occurs as evapotranspiration, base flow to the Mississippi River, and base flow to other streams and to lakes. Possible future response to pumping was studied through electric analog aralyses by stressing the modeled aquifer system in accordance with areal variations in expected well yields. The model interpretation indicates most of the sustained pumpage would be obtained from intercepted base flow and evapotranspiration. Simulated withdrawals totaling 18,000 acre-feet of water per year for 10 years resulted in little adverse effect on the aquifer system. Simulated larger withdrawals, assumed to represent denser well spacing, caused greater depletion of aquifer storage, streamflow, and lake volumes, excessively so in some areas. Results of model analyses provide a guide for ground-water development by identifying the capability of aF parts of the aquifer system to support sustained pumping for irrigation. INTRODUCTION PURPOSE AND SCOPE OF INVESTIGATION This report appraises the capability of the ground-water system in the Little Falls area, Minnesota, to provide water for irrigation. Presently (1971), no extensive development of ground water has tegun Dl D2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES in the area. Increasing awareness of the need for irrigation has stemmed from the combination of unreliable precipitation during the growing season and soils of low water-holding capacity. Avers ge annual precipitation in the area is about 25 inches. Though about two-thirds of this falls during the growing season, its distribution during this period is often unfavorable. Soils are mainly sands and loamy sands developed from glacial outwash. Being typically well drained, the soils commonly show a moisture deficit for optimum plant growth in the summer, particularly during July and August. Irrigation would be important to the Little Falls area by supplementing natural pr?cipitation and thereby increasing the productivity of the soils. Several geologic units, each with distinct hydrologic characteristics, underlie the area. Although each is described here generally, the uppermost unit is described more thoroughly. This deposit of glacial outwash sand and gravel is the largest and most readily available source of ground water. To evaluate its overall water-supply potential, the surficial outwash was studied to determine its extent and thickness, appraise its water-yielding capability, and estimate possible effects that pumping may have on the natural water system. In addition, chemical characteristics of the water are described in terms of suitability for irrigation. The results of this study are intended to contribute toward a better understanding and therefore optimum development of the ground-water resources of the area. LOCATION AND DESCRIPTION OF AREA The study area consists of about 180 square miles in Morrison County, central Minnesota (fig. 1). It is limited chiefly by the extent of surficial outwash, partly by county boundaries, and partly by Camp Ripley Military Reservation boundaries. The study area is relatively low lying, is flat to moderately undulating, and drains generally southward. It forms part of the MississippiSauk watershed unit, as designated by the Minnesota Department of Conservation (Minnesota Division of Waters, 1959). The Mississippi River flows through the study area and is regulated at two locations by dams, one at Little Falls and one northwest of Royalton (Blanchard Dam). Most of the eastern part of the area is drained by the Platte River, which joins the Mississippi River near the southernmost edge of Morrison County. Agriculture is the principal economic activity of the region, although much land is presently out of production. Tl x ? major crops produced are hay, corn, and oats. A small part of the study area is wooded, and a small part consists of lakes and swampr. IRRIGATION, LITTLE FALLS AREA, MINNESOTA D3

Appraisal of ground water for irrigation in the Little Falls area, Morrison County, Minnesota