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I I I I I I I I i I I I I I I I I I I SENATE BILL 96.74 EXECUTIVE SUMMARY I. Introduction Senate Bill 96-74 authorized establishment of a Special Water Committee (SWC) of nine legislators to investigate Denver Basin groundwater management and South Platte River Basin issues. To assist in the SWC's consideration of these water policy issues, the State Engineer (SED) and the Director of the Colorado Water Conservation Board (CWCB) have administered this technical study. The SWC approved a scope of work for the technical study in September 1996 and directed the SED and the CWCB to proceed with the study. The study utilized the latest technical methodologies and data from previous existing studies. During its preparation, the study was subjected to peer review by qualified hydrologists, geologists, engineers, and all interested members of the general public. South Platte River and Denver Basin Overview The pristine South Platte River flow historically was fed from snow melt and ground water (Figure 1). Development within the basin, initially agricultural throughout the basin and urban growth in the upper basin, resulted in the demand for water for agricultural and domestic use exceeding the natural flow of the South Platte River. Due to the increased demands on the South Platte River the management of the water within the basin has intensified over the years. To meet the growing demand, additional water was imported into the South Platte River Basin through trans-basin diversions, primarily from the Colorado River Basin, and groundwater was pumped from the Denver Basin Aquifers. Currently the water supply for the population along the Colorado Front Range, from the Castle Rock area in Douglas County to the Greeley area in Weld County is obtained from a variety of renewable and nonrenewable water sources. The South Platte River annual flow at the Henderson Gage is greater under current conditions when compared to the historic flow. The increased flow is partially a result of wastewater discharge, lawn irrigation return flows from trans-basin diversions and not-nontributary and nontributary ground water pumping. Municipal and industrial water use in the metro Denver area results in about 270,000 acre feet of return flow annually. The pumping of "not-nontributary" and nontributary groundwater also has a depletive effect on the South Platte River. In addition, the urban development in the South Platte River basin has changed the surface runoff characteristics. The increased impervious surfaces and the storm runoff from these surfaces have increased the surface flow. Thus, the water in the South Platte River is now comprised of a number of "types" or colors of water resulting in a net effect of generally more water available for use in the river as compared to the historic flow. This net effect on the South Platte River flow due to (or stemming from) the various water management options and use is demonstrated by the illustrative tool and further discussed in chapter 12. - 1 -