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I I I I I I, I I I I I I I I I il I . I The upper portion of the basin is moderately steep, rolling terrain. The Croke Reservoir located immediately west of Huron Street and the various small ponds along the south tributary, which were constructed as part of the irrigation and stock water supply system prior to urbanization, provide some storm runoff peak flow reduction benefits. Inadvertent detention storage behind the Union Pacific Railroad embankment also provides some flood peak reduction during major storms. The soils of the basin are primarily derived by in-place weathering of the underlying shales and sandstones of the upper portion of the Dawson Arkose, although the soils of the lower reach below Riverdale Drive are largely derived from alluvial deposition from the South Platte River. The shales, sandstones, and conglomerates of the Dawson Arkose Formation, including the Arapahoe Sands, range from approximately 600 feet to over 900 feet thick in the area of the Grange Hall Creek basin. Underlying the Dawson Formation are the laramie and Fox Hills Formations. The upper portion of the Dawson Formation dips gently east-southeast; and due to the relatively impervious nature of some of the shale- sandstone layer in a vertical direction, it is doubtful if a large amount of recharge to the deeper aquifer occurs in this basin. D. Present land Use The present land use in the Grange Hall Creek study area is divided into essentially two parts. The upper basin (west of York Street) is essentially urbanized, except for that portion of Tributary S in Thornton between 104th Avenue and 1-25, whereas the lower basin (east of York Street) is only sparsely developed. The net result on the overall basin is that 54% of the basin is either undeveloped or in the open space land use category. A summary of the present land use for each land use category and each planning basin is presented in Table 11-2. Examining Table 11-2, we find that 48% of Tributary S, 60% of Tributary NE, and 68% of Tributary N are presently developed, while only 10% of Tributary SE and 37% of Grange Hall Creek main stem area are developed. E. Future land Use The probable future land use for the Grange Holl Creek study areo has been determined based upon the existing land use, current zoning of the undeveloped areas, and from the future land use maps and concepts available for Thornton, Northglenn, and Adams County. Incorporating all the input, a future land use map, Figure 11-3, was prepared, reviewed, and approved by the participating entities. The areas of existing zoning that were modified according to the future land use concepts available are shown cross hatched. A summary of the future land use for each category and each planning basin is also presented in Table 11-2. When full development of the basin is obtained approximately 20% will remain open space, with a majority of the open space along the main stem of Grange Hall Creek below the Union Pacific Railroad. About 60% of the basin will be developed into residential areas, with a significant portion of the development in the upper reaches of Grange Hall Creek and its tributaries. In addition to the development aspect of land use, other future land use aspects were considered. The City of Thornton has prepared plans and specifications for the construction of Carpenter Dam immediately west of Colorado Blvd. The effects of the dam on the alternative plans were investigated in Phase A, but after detailed review was deleted as a viable alternative for this study. The City of Northglenn is considering constructing a water supply reservoir at the eastern city limits (NW 1/4 Section 12, Township 2 South, Range 68 West). The Northglenn dam and reservoir, however, was not evaluated for potential flood peak reductions at this time since details on the proposed configuration were n01 available. 6