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", _i.,. "\ '; . . , , ADDENDUM to DEFINITE PLAN REPDRT Narrows Unit Colorado' This addendum report summarizes the present plan and significant developments and changes in the plan which have occurred during advance planning of the Narrows Unit and since the Definite Plan Report (DPR) was completed in May 1973. This report updates costs and benefits and 'reevaluates the economic justification and repayment aspects of the unit. Finally, a chronology and updating reports from participating Federal and State agencies are appended. The unit was authorized for construction and operation by the Congress under Public Law 91~389 on August 28, 1970. The Narrows Unit will be located in northeastern Colorado along the South Platte River as shown on the following area map, exhibit 1. Lands to be served by the unit are in Adams, Weld, Morgan, Logan, Washington, and Sedgwick Counties. The unit will provide supplemen~al irrigation water, flood control, recreation enhancement, fish and wild- life conservation and development, highway improvement, and potential future municipal and industrial water supplies. Features to be constructed for these purposes are Narrows Dam and Reservoir, a fish hatchery and rearing ponds, and several public-use recreation areas. Jackson Lake Dam and Reservoir will be acquired and converted from irrigation use to recreation and fish and wildlife uses. In addition, a 512-mile-long cold water fishery will be established downstream from '; i' rrows Dam. CHANGED RESOURCES CONDITIONS The following changes in resource conditions have resulted in significant modifications to plans and estimates for the unit. Storm and Flood Studies The most significant, change since the DPR was completed was the utilization of the June 1964 Montana storm for final design purposes. This storm and resultant new inflow design flood (IDF) has resulted in major design and structural changes in the spillway for Narrows Dam.