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<br />. standards was approximately $184 million. There are over 10,000 dams in the Rocky Mountains that may
<br />need to be retrofited if the PMP criteria in HMR 55A (Hansen and others, 1988) are used.
<br />
<br />A primary focus of USGS interdisciplinary research is to develop and apply paleoflood techniques to
<br />complement meteorologic, hydrologic, and engineering methods to estimate the magnitude, frequency, and
<br />risk of floods. Paleoflood h~drology is the study of past or ancient floods (Baker, 1987). Kochel and Baker
<br />(1982), Gregory (1983), Baker, Kochel, and Patton (1988), Costa (1987c), Stedinger and Baker (1987),
<br />Stedinger and Cohn (1987), Hupp (1988), and Jarrett (1987, 1990b, 1991) provide summaries of
<br />paleoflood hydrology. Although most studies involve prehistoric floods, the methodology is applicable to
<br />historic or modem floods (Baker and Kochel, 1988; Jarrett, 1990b), and was done here for the Elkhead
<br />River Basin study. Floods leave distinctive deposits and landforms in and along stream channels, as well
<br />as botanic evidence (Jarrett, 1987, 1990b, 1991; Hupp, 1988), Slack-water deposits of sand-sized
<br />particles, flood scars on trees, erosion scars, and bouldery flood-bar deposits commonly are used as
<br />indicators of past flood levels (figure 1). Paleoflood reconstructions provide important information about the
<br />magnitude and age of extreme floods that can be used in risk assessments and provides data to assess
<br />the role of climatic change on flooding and droughts (Jarrett, 1991). Paleoflood data complement and
<br />extend short-term streamflow records and engineering hydrologic methods, and are particularly useful in
<br />providing probable upper limits of the largest floods that have occurred in a river basin (Enzel and others,
<br />1993; Jarrett and Waythomas, in press). Costa (1978) indicated that floods that have happened in the
<br />. recent geologic past are likely to happen in the near geologic future, Maximum paleoflood estimates
<br />incorporate the effects of climatic changes on hydrology during the period of the paleoflood record (Jarrett,
<br />1991).
<br />
<br />In the type of paleoflood investigations conducted in the Rocky Mountains, lack of ohvsical evidence of
<br />the occurrence of floods is as important as discovering tangible onsite evidence of such floods (Jarrett,
<br />1987, 1990b; Jarrett and Costa, 1988). The geomorphic evidence of floods for flows that substantially
<br />exceed bankfull stage in steep mountain basins (Bonner and Stermitz, 1967; Snipes and others, 1974;
<br />Schwartz and others, 1975; McCain and others, 1979; Jarrett and Costa, 1986; Jarrett, 1987; 1990b,
<br />1991; Grimm, 1993; Waythomas and Jarrett, 1994; Jarrett and Waythomas, in press; Pruess, 1996; Brien,
<br />1996) is unequivocal (figure 2). Paleoflood evidence is relatively easy to recognize and long lasting in the
<br />Colorado Rockies, since post-glaciation (10,000 years) or longer in unglaciated basins, because of the
<br />quantity, geomorphology, structure and size of sediments transported and deposited by floods, Glaciation
<br />"erases" evidence of floods, Thus, paleoflOOd investigations can identify physical evidence for the
<br />occurrence or nonoccurrence of substantial out-of-bank or extraordinary floods for very long time periods.
<br />The paleoflood evidence found in many basins in a homogeneous hydroclimatic region defines the regional
<br />maximum flood potential (Enzel and others, 1994; Jarrett and Waythomas, in press), Utilization of an
<br />interdisciplinary study including paleoflood evidence, flood data, and extreme-rainfall data of many river
<br />basins in a region, provides a more complete and accurate assessment of past and potential maximum
<br />. flooding.
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