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<br />estimates developed for areas east of the 105th meridian (Schreiner and Riedel, 1978), which
<br />is about the longitude of Denver, another PMF of 524,000 ft3/s was estimated (Corps of
<br />Engineers, written commun" 1997), Flood-frequency relations for Box Elder Creek have been
<br />of concern to the Urban Drainage and Flood Control District (UDFCD), Federal Emergency
<br />Management Agency (FEMA), and other water-resources agencies, Flood-frequency relations
<br />estimated with different methods in eastern Colorado have large differences, For example, the
<br />100-year flood for Box Elder Creek near Watkins at 1-70 ranges from about 7,900 ft3/s (U.S,
<br />Army Corps of Engineers, 1990) to 26,000 ft3/s (FEMA, 1993). The larger estimate was
<br />based on regional flood-frequency methods developed by McCain and Jarrett (1976), which
<br />have been shown to overestimate flood-frequency relations in eastern Colorado (Livingston and
<br />Minges, 1987; Will Thomas, Michael Baker. Jr" Inc" written commun" 1997), Differences
<br />in flood estimates such as for Cherry and Box Elder Creeks demonstrate the importance of
<br />reducing the uncertainty in estimating the magnitude and frequency of flooding in eastern
<br />Colorado, Perhaps the greatest cause of these uncertainties is the lack of data on extreme
<br />flooding in eastern Colorado,
<br />
<br />METHODOLOGY
<br />
<br />Paleoflood Investigations
<br />
<br />Paleoflood hydrology is the study of past or ancient floods (Baker, 1987), Floods leave
<br />distinctive deposits and landforms in and along stream channels, as well as botanic evidence
<br />(Jarrett, 1990, 1991; Hupp, 1988), Slack-water deposits of sand-sized particles, flood
<br />scars on trees, accumulation of woody-flood debris, erosion scars, and bouldery flood-bar
<br />deposits commonly used as indicators of past flood levels are called paleostage indicators
<br />(PSis), When flows are large enough. streambed and bank materials are mobilized,
<br />transported, and deposited as PSis, The types of river sites where flow competence decreases
<br />and flood deposits commonly are found and studied include: (1) locations of rapid energy
<br />dissipation, where flood sediments would be deposited, such as tributary junctions, reaches of
<br />decreased channel gradient, abrupt channel expansions, or reaches of increased flow depth; (2)
<br />ponded areas upstream from channel contractions; and (3) locations along the sides of valleys
<br />in wide, expanding reaches where fine-grained sediments or slack-water deposits would likely
<br />be deposited, Paleoflood data are particularly useful in providing probable upper limits of the
<br />largest floods that have occurred in a river basin (Jarrett and Costa, 1988; Jarrett, 1990, in
<br />review a; Enzel and others, 1993; Jarrett and Waythomas, in press),
<br />
<br />Paleoflood discharge was determined from estimates of flood width and depth corresponding to
<br />the height of PSis and channel slope for each cross section obtained during onsite visits to
<br />streams. The slope-conveyance method (Bames and Davidian, 1978) was used to estimate
<br />paleoflood discharge, Flow-resistance coefficients and velocity were estimated from analysis of
<br />data for Colorado rivers (Jarrett. 1985), Analysis of the differences in PSis and highwater
<br />marks (HWMs) of extreme floods in the past 3 years for 122 sites in 80 streams of the
<br />western United States indicated that the elevation of the top of flood-<Jeposited sediments (PSis)
<br />generally are within +/-0,2 ft of flood HWM elevations (Jarrett, in review b), Therefore. use
<br />of the top of flood-deposited sediments as PSis for streams in this study provides a reliable
<br />estimate of the maximum paleoflood depth that is then used to reconstruct the discharge of
<br />paleofloods,
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