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CHAPTER I--INTRODUCTION

1. General Description. - Trenton Dam is located on the Republican River about 2-1/2 miles west of Trenton, Hitchcock County, Nebr. The dam is a rolledearth structure approximately 8, 600 feet long and rises 144 feet above the lowest point of the excavation. A spillway with a maximum discharge capacity of 126, 500 secondfeet, constructed through the left abutment, protects the structure from possible flood damage. A river outlet is provided through the spillway structure for river regulation. At the opposite end of the dam another outlet works is provided to meet irrigation requirements. This outlet works forms the headworks for the Meeker Irrigation Canal. An aerial view of Trenton'Dam is shown in figure 1.

The dam is one of the key storage and flood control features of the Missouri River Basin project. Among the benefits derived from the construction of Trenton Dam are flood control, diversion for irrigation, storage, sedimentation control, fish and wildlife preservation, and recreation.

2. History and Authorization. - The extreme drought of the 1930-1940 decade focused nationwide attention on this and surrounding areas in Nebraska and the midwest. Periodic floods with accompanying loss of lives and property damage in the Republican River Valley stimulated further interest of State and local groups in providing protective measures against nature's elements. In the flood of 1935, over 100 lives were lost and much property destroyed. Heavy property damage was also caused by a series of floods in 1947.

The Frenchman-Cambridge division, of which Trenton Dam is part, was authorized as an approved division of the Missouri River Basin development plan by the Flood Control Act (Public Law 534) of December 22, 1944, 78th Congress, 2nd Session. Enders Dam and Medicine Creek Dam, both Bureau dams, are also parts of the Frenchman-Cambridge division. Enders Dam is located near Enders, Nebr., and Medicine Creek Dam 8 miles northwest of Cambridge, Nebr. This development plan was outlined in Senate Document No. 247 as a coordinated plan of the Bureau of Reclamation and the Corps of Engineers, U. S. Army. In July of 1947, funds were appropriated by Congress for an accelerated flood control program, including a specific appropriation of $300, 000 for early construction of Culbertson Dam and Reservoir. The names of these features were changed to Trenton Dam and Swanson Lake, respectively, by Congress upon enactment of Public Law 61 on May 12, 1949.

3. Alternative Sites. - A tentative dam site was selected on the Republican River about 8 miles east of Trenton, Nebr., and 2 miles west of the confluence of Frenchman Creek and Republican River near Culbertson, Nebr. Extensive studies and exploration of the dam site were accelerated by the Congressional action in 1947. Results of studies disclosed unfavorable foundation conditions, a higher than anticipated railroad relocation cost, and necessity for a protective dike at the head of the lake for the town of Trenton. Alternative sites west of Trenton were studied and a site 2 miles west of Trenton was finally selected as the best suited topographically and geologically, which also afforded the most economical railroad relocation. (See fig. 2.)

4. Geology. - In the vicinity of Trenton Dam sedimentry deposits extend to a depth of about 7, 000 feet and rest on pre-Cambrian granites. The lowest and oldest formation exposed during dam construction was Pierre shale, which at the damsite has a thickness of about 1, 000 feet. This shale, deposited during the Cretaceous period, is overlain in the river valley by alluvial sands and on the valley abutments by Ogallala sand and gravel deposits. These deposits at both abutments are capped by a Loveland loess strata about 15 feet thick which is overlain by Peoria loess that forms the upper and surface layer of the region.

The loess encountered at both abutments is composed primarily of fine, loosely cemented, angular grains of fine sand admixed with calcareous particles and small amounts of clay and organic materials. It has low unit weight and varies considerably in density because of the haphazard arrangement of particles and inconsistent amounts of cementing clays and calcareous materials. The average mineralogical composition of samples taken from the abutment is as follows:

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