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I JARREIT AND TOMUNSON: REGIONAL INTERDISClPUNARY PALEOFLOOD METIlOD .~x:.~~' Y,"'\:' ~W Colo. ~ ..~~I' . II: . I lC" lit!. ...... .~:l x . .1 X XXX ........."""YJi -11M)! ! NWColo. .. .~. I( '\r... j'k. (contemponry) .A)( Il.lClC. .. I . lAx lI4I: ')(.11.. Ix '''x ",oX III .x'k. ! . M l( . I( "0 i .x .. ~. lit ~ f . I ~ II IIi; lC lC]I( . Ie X a: j Ie 'tJ l i'-x I( ! ! )( !i! i jjj j. 10000 ..,. United States E. Colorado (Costa) <2)300 m (Jarrett) 1000 100 .!!! ~ e ,; 10 !!' . ~ 0 . is .. . . .. .1 . .0' .1 10 Drainage area, km2 2975 . III gage . ungaged site . plIleoflood <5000 yrs '" paleoflood >5000 yrs 100 100000 1000 10000 Figure 9. Relation between contemporary and paleoflood peak discharge and drainage area with envelope curves for northwestern Coiorado. Envelope curves of maximum flooding for eastern Colorado [Jarrett, 1990b] and for the United States [Cosla, 1987a] are shown for comparison. vegetation cover, and infiltration rate [Gilley el al., 1993]. For example, as little as 25 to 50 mm of rain in a few hours can produce rill erosion on bare, poorly drained soils on steep slopes [Hadley and Lusby, 1967; McCain el al., 1979; Jarrett, 1990b; Jarrett and Browning, 1999]. Thus a lack of rill erosion is a good indicator that intense rainfall is uncommon or that erosion healing rates arc high. Extensive rill and deep gully erosion are common in arcas subject to intense rainfall. For example, gullies up to 2 m deep formed on hillslopes in the Big Thompson River basin where rainfall exceeded 150 mm in a few hours during the rainstorm of Juiy 31, 1976 [McCain 01 aI., 1979]. Jarrett and Browning [1999J used geomorphic techniques to relate hillslope erosion with rainfall data for an extreme rainstorm on July 12, 1996, in Buffalo Creek, located in the foothills near Denver. Their geomorphic estimated maximum hourly rainfall of 115 mm, which was determined immediately after the storm, compared with 130 mm independently derived in 1998 from Doppler radar signatures and upper air observa- tions [Henz, 1998]. Part of the subjectivity in using billslope erosion is estimating the time rills and gullies remain [Jarrett and Browning, 1999]. Rill and gully networks formed during extreme rainstorms such as in 1965 and 1976 in eastern Colo- rado [McCain el al., 1979; Mallhai, 1969] and west central Colorado [Jarrett, 1990b] have changed iittle in the intervening years. Conversely, in regions of the Rocky Mountains not sub- ject to intense rainfall, there is a general sparsity of hillslope erosion evidence on slopes where evidence should have been preserved had large rainstorms occurred. On.site inspection indicated that rills and gullies are small or nonexistent in basins above about 2000 m in northwestern Colorado. Lack of rilling throughout such a large area provides additional supporting evidence of the absence substantial rain- storms in recent times. Hillsides having sparse vegetation and comprised of sand or finer-grained soils such as in Piceance Creek and Yellow Creek basins have rill and gully erosion, but these basins are in the far southwestern part of the regional study area. However, no hillslopes have gully development similar to basins at lower elevations in eastern Colorado that are subject to large, intense rainstorms. Rainfall-frequency relations deveioped for Colorado [Miller el aI., 1973] can be used to assess the frequency of contempo- rary rainfall data. Superimposed on Figure 8 are the 10-year and 100-year, 24-hour duration rainfall frequency relations developed from Miller el al. [1973] along an east-west transect from the crest of the Park Range to Maybell. For comparative purposes the 6-hour and 24.hour PMP estimates for Eikhead Reservoir are shown on Figure 8. Hansen el al. [1977, Figure 5.7] compared the ratio of the 24-hour PMP estimates to 100- year, 24-hour rainfall frequency estimates for the western United States (e.g., for Coiorado using Miller el al. [1973]); they sugges1 reasonabie ratios between 2.8 and 5. For E1khead Reservoir the ratio is 8.4 (510 mm/61 mm). Although there may be some uncertainty in estimating rainfall frequencies, the ratio adds further support to the conclusion that PMP esti- mates for the Colorado Rockies may be too large. High moun- tain barriers (Figure 1) reduce the available atmospheric mois- ture from the Pacific and Gulf of Mexico to northwestern Colorado [Tomlinson and Solak, 1997]. 5.2.2. Maximum /looding. Records from 198 streamflow- gaging stations in northwestern Colorado,' primarily in the Yampa River and White River basins, were analyzed; some have peak-flow data since the early 19oos. These gages are fairly uniformly distributed in the study area. To help define the maximum flood potential for northwestern Coiorado, flood data from 20 ungaged sites that define maximum flooding from intense, localized rainstorms in northwestern Colorado [Jarrett, 1987, 1990b; data available at http://waterdata.usgs.gov), also were incorporated into the database. Maximum peak discharge is 940 m3 S-I, drainage areas ranged from 0.21 to 19,840 km2, gage elevation ranged from 1595 to 3200 m, and there were a totai of 3512 station years of record. A reiation of maximum discharge, including paleoflood data, and drainage area with the envelope curve for northwestern Colorado is shown in Figure 9. The largest gaged rainfall.produced flood of 190 m3 I i' .. I, " .