FLOOD06290

• County: Statewide
• Basin: Statewide
• Title: Reliabilty of Paleostage Indicators for Paleoflood Studies
• Date: 3/20/1998
• Prepared By: USGS/BOR
• Floodplain - Doc Type: Educational/Technical/Reference Information

draft 3/20/98

We expanded our analysis by looking at older extreme floods in semiarid and arid climates where

HWM evidence can be preserved for many decades after a flood (unpublished data). These include

the: 1965 Plum Creek flood (Matthai, 1969), about a 1,200-yr flood (Osterkamp and Costa, 1987);

1976 Big Thompson River flood (McCain et al., 1979), about a 1O,OOO-yr flood (Jarrett and Costa,

1988); 1982 Lawn Lake dam-failure flood (Jarrett and Costa, 1986) in Colorado, where the discharge

exceeded the 500-yr flood by a factor of about 30; 1990 flash flood near Opal, Wyoming, which is

one of the largest flash floods in Wyoming (Corrigan and Vogel, 1993); and several streams in the

Wasatch Mountains of Utah, which had discharges greater than l00-yr floods (Christensen et al.,

1991), The PSI-HWM data from a range of river settings are representative of evidence typically

preserved for extreme paleofloods.

Results and Discussion

The surveyed cross-section data at each site (e.g., figs. 2a, 2b, and 3) were used to determine the

difference between the elevations at the top of the flood deposit (PSI) and the HWM at the flood

margins for 131 sites in 90 streams. Flood sediments were deposited as mid-channel and point bars

in the channel and on the floodplain and slack-water sediments were deposited along channel

margins. In almost all cases, the maximum elevation at the top of flood-deposited sediments (new

PSIs) closely matches the flood HWM elevations (figs. 4 a-f). No statistically significant relations

exist between the difference of PSI-HWM and explanatory variables. The average difference between

PSI-HWM elevations for all data is +0.06 m (+5 percent of depth); for finer-grained sediments

(SWDs) is -0.025 m (-5 percent of flood depth); for bouldery-flood bars (FBs) not including 1982

Lawn Lake dam-failure flood deposits is about +0.2 m (+15 percent); and for the Lawn Lake deposits

is about +0.4 m (+12 percent).

Bouldery flood mid-channel and point bars, which form in higher energy areas, explain the few cases
where the tops of the sediments protrude above the HWM elevations. These bars form either as new
bars around an obstruction (e.g., a boulder) or when sediments roll up existing bars and cease motion
when flow depth decreases. Record snowmelt flooding in 1995 from many streams in Colorado

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