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<br />Occurrence of extreme floods can be validated with precipitation data (and vice versa),
<br />Integrating precipitation, streamflow, and paleoflood information with a rainfall-runoff model
<br />leads to a better understanding of the physical processes that cause extreme floods (Cudworth,
<br />1989), Although rainfall-runoff models are used for calibrating extreme storms/floods, the task
<br />committee will not cover this subject in the monograph,
<br />
<br />Historical data are useful in extending streamflow records of peak discharge, The data
<br />are based on observations of the most extraordinary flood events, so the validity and reliability of
<br />the observations need to be determined, Historical information may come from newspaper
<br />articles, eyewitness accounts, or other reports. The data are usually incomplete, but may record
<br />information about flood thresholds, exceedances, or nonexceedances. The length of record
<br />varies according to the period of time people have occupied the area and their ability to preserve
<br />the information (England, 1998).
<br />
<br />Paleoflood Data
<br />
<br />Paleoflood data provide flood information at ungaged locations that can be used with
<br />systematic and historical records to extend the period of record typically up to about 10,000
<br />years (Kochel and Baker, 1982; Baker, 1987; Jarrett, 1991), The monograph will describe the
<br />types of physical evidence that establish paleostage indicators, Fluvial geomorphic evidence
<br />includes erosional and/or depositional features that are used to infer paleostages or non-
<br />inundation levels (England, 1998), Botanical evidence uses vegetation to record flood levels and
<br />can be used as a measure of stability of a geomorphic surface (Hupp, 1988),
<br />
<br />Paleostage indicators (shown on Figure 1) resulting from fluvial geomorphic evidence
<br />include slack-water deposits, scour and silt lines, gravel bars, and geomorphic surfaces. This
<br />evidence is usually found in channel reaches that are rapidly expanding or flattening, tributary
<br />areas, and ponded areas. Slack-water deposits of sand-sized material accumulate in channel
<br />reaches where flow velocities substantially decrease, Scour lines typically occur where flow
<br />velocities are sufficiently high to erode geomorphic surfaces,
<br />
<br />Terraces and alluvial fans are geomorphic surfaces that can be used to establish discharge
<br />bounds and the frequency of flooding. Surfaces that have been innundated and eroded provide
<br />an indication of the number of times a particular stage has been equaled or exceeded over time
<br />(Levish et aI., 1994), Likewise, knowing that a geomorphic surface has not been flooded
<br />establishes an upper bound on the peak discharge for a particular period of time that can be used
<br />in flood frequency analysis (Jarrett and Costa, 1988).
<br />
<br />Botanical evidence uses tree rings, scars, and the age structure of riparian vegetation to
<br />record evidence of past floods and to date floods (Hupp, 1988; Gottesfeld, 1996), Large floods
<br />carry large floating debris that scar trees, and have high velocity flows that uproot riparian
<br />vegetation, Tree scars indicate the maximum flood stage, Tree rings are used to date the flood
<br />by counting annual growth rings, The age structure of riparian vegetation may be used to date
<br />the last innundation of a geomorphic surface or to indicate stability of the surface,
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