The Mississippi River Valley alluvial aquifer is a major source of water for irrigation in much of eastern Arkansas. Maps of the potentiometric surface, water level change, and depth to water illustrate the effects of large withdrawals for irrigation on water levels in the aquifer. The Mississippi River Valley alluvial aquifer is composed of Quaternary Flood plain and terrace deposits. Water in the aquifer generally is confined by an upper silt and clay layer. The regional direction of groundwater flow is south and southeast, except where affected by large withdrawals for irrigation by stream reaches that are hydraulically connected with the alluvial aquifer. The largest well yields range from 1,000 to 3,000 gal/min. Recharge to the aquifer principally is by infiltration of precipitation and water for surface water bodies. Both the spring and fall potentiometric surface maps show extensive cones of depression that result from large withdrawals for irrigation-one centered beneath Arkansas County, and another beneath western Poinsett and Cross Counties. Average water level differences between the spring and fall indicate a general decline of about 3 ft. The largest declines in water levels between 1981 and 1986 and the deepest water levels in 1986 occurred in areas most affected by large withdrawals for irrigation, as shown by the water level change map and depth to water map. The largest water level declines mainly occurred in central Lonoke County and in western Poinsett County. (USGS)