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2960 . JARRElT AND TOMLINSON: REGiONAL INTERDiSCIPLINARY PALEOFLOOD METI10D t tfeme precipitation in the mountains of Wyoming. Tomlinson.~ and Solak [1994, 1997] developed a site-specific methodoiogy to determine PMP/PMF estimates. Jensen [1995] developed new criteria for computing PM? estimates for short-duration. small-area storms in Utah. His study resulted in significant decreases in extreme precipitation for Utah compared to cur. rently used PMP estimates [Hansen el aI., 1977J. Large differences in estimates of extreme rainfall and flood- ing have substantial effects on dam safety. For example, a paieoflood study was conducted for the Bureau of Reclamation for Olympus Dam in Estes Park, Colorado, on the Big Thomp- son River [JaITell and CosIo, 1988]. Olympus Dam is iocated at an elevation of 2300 m, and the spillway was designed for a PMF of 637 m' S-I. However, a revised PMF (Bureau of Reclamation, written communication, 1988), based on the re- vised PMP estimates [Hansen el 01., 1988], is 2380 m' S-I. Paleoflood investigations by Jamll and COSIo [1988J indicated that the largest natural fiood flow in the Big Thompson River upstream from Olympus Dam is 142 m' s" (6% of the revised PM F) during at least the past 10,000 years (since glaciation). This paleoflood information and a review of the existing and revised PMF values by the Bureau of Reclamation resulted in a decision not to modify the spillway for Olympus Dam at an estimated cost of $10 million (Bureau of Reclamation, written communication, 1988). In part because of the interdisciplinary research, concerns and questions of extreme rainfall and flood design values for structures located in floodplains have been raised by state and fcdcral dam safety officials for the Rocky Mountains. Most state agencies in the Rocky Mountain region have ongoing hydromctcorologic and paleoflood studies to revise methodol- ogies to estimatc extreme precipitation and flooding for dam safety because of recognizcd deficiencies in PM? estimates in mountainous areas. Colorado began studies to develop new methods to estimate extreme rainfall in the mountains [McKee alld Doesken, 1997], and the sccond, 30-month phase recently began (A. Pearson, Colorado Dam Safety Office, written com- munication, 1999). The Bureau of Reclamation recently began a program to use a risk-based assessment, which incorporates paleoflood investigations to provide estimates of the magni- tude and frequency of extreme floods, to assist with dam safety decision making [Levish elal., 1994; OSlellaa and Levish, 1995J. The U.S. Army Corps of Engineers is impiementing a risk assessment method to evaluate potential safety problems for its more than 550 dams to aid decision makers in prioritizing investment decisions [Foster, 1999]. In 1999 the American So- ciety of Civil Engineers began a task committee on paleoflood hydrology as it relates to dam safety and risk-based assess- ments. The NRC [1988, p. 111] states that "Nonetheiess, the expense of such studies is minor in relation to planning costs for major high-risk projects such as nuclear power plants or large dams. At present these opportunities are largely being ignored. . . . For critical projects the paieoftood data should at least be collected, appropriately weighed, and considered in the overall decision process leading to design." Thus it is im- portant to develop methodologies that can be used by dam safety officials to make decisions about the probabilities of extreme floods. 3. Study Area The Yampa River in northwestern Colorado originates on the White River Plateau (aiso known as the Flattops with a maximum elevation of 3808 m) and flows westerly through the Gore (3295 m), Rabbit Ears (3748 m), and Park (3725 m) moun1ain ranges (Figure I). The White River originates on the White River Plateau and also flows westerly. The boundary between,northwestern and southwestern Coiorado is defined by the topographic divide between the White River and Col- orado River basins (Figure 1), which has elevations ranging from about 2500 to 3800 m. Elevations at the downstream study limit are 1804 m at Maybell in the Yampa River basin and 1898 m at Meeker in the White River basin. Major Yampa tributaries include the Elk and Uttle Snake Rivers and Eik- head and Fortification Creeks. The regionai study area is ap- proximately 10,900 km'. ElkheadCreek has its headwaters in the Eikhead Mountains and flows southwesterly to its confluence with the Yampa River about 10 km east of Craig (Figure I). Elkhead Creek basin has a drainage area of 531 km' at Elkhead dam. Eieva- tions in the basin range from about 3307 m at the highest peak of the Elkhead Mountains to about 1890 m at its confiuence with the Yampa River. The elevation of Elkhead Reservoir is about 1950 m. Distinct mountains and ridges define the north (-2900 m), east (-2400 m), and west (-2300 m) boundaries of the basin. The topography is rolling hills and valleys, except in the steeper, mountainous headwater areas. Elkhead Creek and numerous tributaries drain the mountains forming the basin boundary. Most streams in the study area are of bigher gradient with slopes greater than 0.002 m m-1 [Jamll, 1984], except the lower reaches of Elkhead Creek and the lower Yampa River. Cobble- and boulder-sized material make up the stream bed and fine.graioed sediments compose the fiood- plain. Some lower-elevation tributary valleys to the Yampa and White Rivers are predominantly fine-grained alluvial fill. Elkhcad Creek basin is underlain by Cretaceous and Ter- tiary rocks (shale, sandstone, conglomerate, and coals) in the Lance, Lewis, Wasatch, Browns Park, Fort Union, and nes formations; upper Tertiary intrusive rock, primarily porhyries of intermediate and basaltic composition, cover parts of the basin [Twe!o, 1976]. Within the generai study area, similar geologic fonnations as in Elkhead Creek basin with Precam- brian rocks (granite, Quartz monzonite, granodiorite, Quartz diorite, and gabbro) and biotite and hornblend gneisses occur in the Park Range. Tertiary andesitic and basaltic lava flows from the Flattops and Elkhead Mountains with some intrusive rocks in the study area [Twelo, 1976]. Most of the Park Range, upper Elkhead Creek basin, and the Flattops experienced at least three Pieistocene giaciations [Madole, 1982, 1989, 1991a, 1991b, 199Ic]. Rights of Pieistocene terraces along the Yampa River from .about Steamboat Springs downstream to about Craig, probably from glacial processes, ringe from about a meter (early Holocene) to 183 m (620 ka) above the present f1oodpiain; the average incision rate since 600 ka is 0.11 m ka-1 [Madole, 1991a]. Ungiaciated tributaries lack the well- developed Pleistocene terraces, and Holocene terraces are rel- atively close to the valley floors [Madole, 1991aJ. Loess, typi- cally 1.3 to 2 m thick, of at least two ages is widespread in the Yampa River basin with the latest deposition in the late Pleis- tocene and possibly early Holocene [Madole, 199Ia]. The majority of Elkhead Creek basin and regional study area has been mapped as low to moderately well-drained soils, except in limited higher elevation areas where bedrock is at or near the ground surface [Soil Conservation Service, 1982; Nat- ural Resol/rces Conservalion Service, 2000]. At higher elevations in Elkhead Creek and the upper Yampa River and White t' ? ;/ .{ ~