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<br />Squaw Creek, and Cedar Creek near Cedar Creek and their tributaries (figures 1 and 2). Joe's
<br />conclusion that there is little evidence of substantial flooding (1952 or other years) in the Cimarron area is
<br />correct. The river gradient of the Cimarron River at the paleoflood sites averages about 0,01 ftJft,
<br />Paleostage indicators (PSI) corresponding to the maximum paleodischarge consist of cobble and bouldery
<br />flood bars and piles of large woody debris, which appeared to be about 20- to 50-years old (figures 3a
<br />and 3b). The high-water marks (HWMs) from peak flows in 1995 were about 2 feet lower than the
<br />maximum PSis (figure 3c).
<br />
<br />Generally, flood bars along the Cimarron River are relatively small and have low relief as compared to
<br />bars in streams subject to large flash floods. The largest flood bars, which are located in the downstream
<br />reaches of the Cimarron River near Cimarron, were selected as the paleoflood-reconstruction sites, To
<br />estimate the maximum paleoflood, cross sections and PSis were surveyed at four sites on the Cimarron
<br />River between Cimarron and the location of the 1952 rain gage, plus one site near its confluence with the
<br />Gunnison River. Maximum-particle size available for transport in the channel is 10 feet in diameter.
<br />Maximum-particle size on flood bars are 2,5 feet. For comparison, flash floods in small mountainous
<br />basins in Colorado have transported boulders ranging in size from 2 to 25 feet (Follansbee and Sawyer,
<br />1948; McCain and others, 1979; Jarrett and Costa, 1986), which demonstrates the flow competence of
<br />floods, That streamflows in the Cimarron River only are capable of mobilizing and transporting particles
<br />up to 2 feet indicates that substantial flooding has not occurred. The flood-bar deposits in the Cimarron
<br />River are relatively well sorted and imbricated (preferred orientation of platy clasts) (Costa and Jarrett,
<br />1981; Jarrett and Waythomas, in press). In most rivers, imbricate and sorted clasts often result from
<br />sustained flows. Sustained flows result from either snowmelt runoff or a long duration rainfall-runoff event
<br />that has a gradual decline of discharge. A gradual decline of discharge results in a decrease of sediment
<br />transport potential, and thus, selective deposition of transported sediments. Conversely, flash-flood
<br />deposits are poor1y to very poor1y sorted because of a rapid decrease in discharge. However, flash-flood
<br />deposits also are imbricate. Thus, sedimentologic evidence in the Cimarron River suggests snowmelt-
<br />runoff dominated sediment transport, If intense, short-duration rainstorms and associated flashy runoff
<br />produced sediment transport in the Cimarron River basin, subsequent larger snowmelt peak flows would
<br />have reworked these smaller rainfall-runoff deposits,
<br />
<br />Paleodischarge estimates were made using the slope-conveyance method (Barnes and Davidian,
<br />1978; Chow, 1959; Davidian, 1984) associated with the highest PSI (top of flood bars) in the Cimarron
<br />River. Manning's flow-roughness coefficients (Chow, 1959) were used to quantifying flow resistance.
<br />These coefficients were estimated using methods developed from an analyses of verified flow-resistance
<br />data for mountain rivers in Colorado (Jarrett, 1984, 1985) during high-flow to flood conditions (Jarrett, 1986;
<br />Trieste and Jarrett, 1987). The maximum paleoflood discharge in the Cimarron River ranges from 3,000 to
<br />4,000 cubic feet per second. The drainage area at the paleoflood sites averages 220 square miles. Thus,
<br />the maximum unit discharge is about 18 cubic feet per second per square mile.
<br />
<br />REPRESENTATIVENESS OF PSIS OF HWMS
<br />
<br />During flooding, erosional and deposition features occur that modify vegetation and morphology of flood
<br />plains, and catastrophic channel changes can occur during extreme flooding, In paleoflood studies of
<br />mountain rivers, the most commonly used paleostage indicator (PSI) is the elevation of the top of flood
<br />bars deposited by floods (Jarrett, 1986; 1987a, 1990, 1991; Jarrett and Malde, 1987; Jarrett and Costa,
<br />1986,1988; Jarrett and Waythomas, in press). One valid question of past paleoflood estimates has been
<br />the representativeness of using the top of flood bars (or PSis) as the minimum height of past flooding.
<br />Snowmelt runoff in 1995 established record maximum peak flows at many streamflow-gaging stations in
<br />Colorado. Some streamflow-gaging stations in Colorado have been operated for about 100 years, In
<br />1995, many Colorado streams had flows having a recurrence interval between 25 to 100 years, and a
<br />number streams had flows with recurrence intervals in excess of one hundred years. The 1995 flows
<br />caused substantial sediment mobilization and transport in many streams in Colorado. Deposition of these
<br />sediments produced new flood bars or enlarged existing flood bars; several bars were observed forming
<br />during peak-flow conditions (figures 4a and 4b). Thus, 1995 provided an opportunity to assess the
<br />actual relation between the elevation of high-water marks (HWMs) and PSis.
<br />
<br />Onsite visits to were made to streams throughout Colorado during the high-flow season of 1995, which
<br />lasted from about April through July. After snowmelt runoff receded, extensive post-flood surveys were
<br />made. Forty cross-section and 1995 water-surface elevation surveys of the new flood bars were made
<br />on 34 streams in Colorado. In addition, PSI-HWM data from several pre-1995 large floods also were
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