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1 CHAPTER 3 1 RESERVOIR SITE CHARACTERIZATION 1 This chapter describes important reservoir site characteristics that bear on technical and cost factors of enlarging Elkhead Reservoir. The discussion is divided into four broad areas: land use and ownership, geotechnical investigations, flood hydrology, and sediment movement. ' LAND USE AND OWNERSHIP General Physiographic Setting Elkhead Creek is located in northwest Colorado and is tributary to the Yampa River just east of the City of Craig. Elkhead Reservoir is located three miles upstream of the confluence of Elkhead Creek with the Yampa River and straddles the Moffat/Routt county line, as shown on Figure 3-1. The reservoir is contained behind an eighty-foot high, 1,160-foot long earthfill ' dam across Elkhead Creek. The Elkhead Creek headwaters are along the Wyoming border in the Elkhead mountains which contain Bears Ears Peaks as the distinguishing land feature. Elkhead Creek flows from a subalpine environment of moderate density pine forest at an elevation exceeding 10,500 feet to a sage brush/grass dominated environment of rolling hills at an elevation of 6,300 feet at the dam site. The drainage basin area above the dam is 205 square miles with water collected into the major tributaries of Dry Fork Elkhead Creek and North Fork Elkhead Creek. The nearest representative streamflow gage to the dam is located near the Town of Elkhead (USGS Station No. 09245000, drainage area 64.2 me). High annual streamflow occurs generally between mid-April and mid-June as a result of snowmelt. At the dam the annual peak flow is approximately 600 cfs, occurring around May 20, and flow decreases to less than 10 cfs by late ' in the year. Historical Land Development and Existing Land Use The historical and existing land use in the reservoir area has been largely dictated by climate and the natural resources of the area. Archaeological evidence indicates only sparse occupation of the area by native peoples prior to European settlement. Following the initial Spanish exploration of the area in 1776, European activity through the 1870's was limited to modest trapping, exploration and livestock raising activity. Explorers Fremont and Powell visited the region but the area remained dominated by the Northern Ute Indians until they were evicted to a reservation in Utah in the early 1880's. Communities such as Craig (established in 1889) developed around cattle, sheep and speculative homesteading industries but the area remained sparsely populated due largely to lack of adequate transportation facilities. The railroad in the 1890's and U.S. Highway 40 in 1920 opened up the area but it continued to be characterized by a relatively slow population growth. 3-1