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In the area north of Boulder, the dip is gradual and the aquifers rise to the surface in a 2- to 7-mile-wide band of outcrop that extends to the northwest and southwest of Greeley (fig. 1), This large outcrop band is an important recharge area for the bedrock aquifers, for here rain and snowmelt can perco- late to the aquifer over a large area and recharge the ground-water supply. In the area south of Boulder, the bed- rock aquifers have been more severely SHALLOW AQUIFERS deformed by movement on mountain- front faults, and the exposed beds of permeable sandstone dip almost vertically into the subsurface, In this area, the outcrop band is narrow, rang- ing from a few hundred to a few thou- sand feet in width, This outcrop is a minor recharge area because of its small extent. Where stream valleys cross the outcrop band of the bedrock aquifers, permeable sand and gravel of the shal- low aquifer in the stream valley may be in direct contact with permeable sandstone of the bedrock aquifers, In these areas, called subcrop areas, ground water generally flows from the bedrock aquifer to the shallow aquifer. However, pumping may reduce the water level in the bedrock enough to cause water to flow from the shallow aquifer to the bedrock in some areas. Demonstration area oWellington Forto Collins Loveland 0 o Greeley Platteville / , . oLongmont .- Fort L9{lton '.- Bii hton oBoulder Comprehensive mapping of the thickness, extent, paleotopography, water-table altitude, saturated thick- ness, and depth to water in the shallow aquifers in the 680-square-mile Denver metropolitan area has been completed (Robson, 1996). This work indicated that the alluvial sediments of the principal aquifers generally become thinner and coarser grained closer to the front range of the Rocky Mountains, Sediment thickness ranges from less than 20 feet near the moun- tains to 40 to 60 feet in the downstream reaches of the South Platte River near Brighton, Y 111 If ~MllES o 1 2 3 KILOMETERS Figure 2. The present valley of the South Platte River is near Brighton. An ancient paleovalley formed by the ancestral South Platte River extends northeastward under Barr Lake (Robson, 1996, HA-736), A map of the paleotopography in Robson (1996) (fig, 2 is a part of this map) shows the altitude and configura- tion of the buried bedrock surface, The metropolitan area map shows the loca- tion of the current stream valleys, which are cut into the bedrock surface, and the location of several paleovalleys that were cut into the bedrock surface by ancient streams which no longer flow through the areas of the paleovalleys, The largest of these paleovalleys is 1 to 2 miles wide and extends northeast- ward under Barr Lake, This paleovalley likely was formed by the ancestral South Platte River before it changed course and cut its present deeper valley near Brighton, Alluvium in the pale- ovalley is as much as 60 to 90 feet thick. Ground water still flows down the trend of the paleovalley, although there is no natural stream in the , present valley. Saturated thickness of the shallow aquifers is the distance from the water table to the base of the aquifer, In most of the upland areas between stream valleys, saturated thickness is less than 20 feet. However, the areas of greatest saturated thickness (60 to 80 feet) are in the Cherry Creek Valley, in the South Platte River paleovalley, and in several other smaller paleoval- leys in the metropolitan area (Robson, 1996), Depth to the water table gener- ally is less than 40 feet in most of the metropolitan area and is less than 10 feet in the central part of the larger stream valleys, The general shallow depth to water and permeable soils enable ground-water recharge to occur over a broad area, Ground water flows from the upland areas toward the nearby stream valleys where the water flows down the valley and toward the stream, Ground water discharges from the aquifer by flowing into streams, by withdrawal from wells, or by evaporation and transpiration from vegetation, )