FLOOD04278

• County: Jefferson
• Community: Morrison and Evergreen
• Stream Name: Bear Creek
• Basin: South Platte
• Title: Coarse-sediment Distribution as Evidence of an Elevation Limit for Flash Flooding
• Date: 12/8/1995
• Prepared For: CWCB
• Prepared By: Elsevier Science Publishers
• Floodplain - Doc Type: Flood Documentation Report

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RECEIVED
DEe 0 8 1995

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ELSEVIER

Colorado Water
Conservation Board
Geomorphology 00 ( (995) 000-00o

Coarse-sediment distribution as evidence of an elevation limit for
flash flooding, Bear Creek, Colorado

Michael M. Grimm I, Ellen E, W ohl, Robert D. Jarrett
Department of Earth Resources. Colorado State Universiry, Ft. Collins. CO 80523. USA
U.S. Geological Survey, Water Resources Division. Box 25046, MS 418, Denver Federal Center, Denver. CO 80225. USA

Received to October 1994; revised 18 Febru::rry 1995; accepted 3 March 1995

Abstract

Bear Creek is a tributary of the South Platte River in central Colorado. The stream flows east from an elevation of 4348 m at
the Continental Divide to the mounlain front at 1670 m. It thus encompasses the 2300 m elevation limit for substantial rainfall
flooding io tbe Colorado Front Range proposed by Jarrett. Maximum paleoflood discharges estimated from flood deposils al
four sites along Bear Creek demonstrate a consistent decrease in unit discharge with increasing elevation and support the
hypothesis of an upper elevation limit for rainfall floods. The unit discharge values were used to explain coarse-sediment
distribution along Bear Creek. Measurements of coarse. grained channel sediment at 19 sites along the creek indicate a decrease
in particle size in flood deposits with increasing elevation. as well as a decrease in the size of clasts introduced to the main
channel along tributaries. These changes in grain size are hypothesized [0 reflect changes in the competence of Channel transport
as a result of snowmelt-dominated versus rainfall.dominated discharge regimes above and below 2100 m elevation. Calculations
of flow competence versus entrainment thresholds for the deposita may support this interpretation. One of the geomorphic
implications of the elevation limit on flash ft.ooding is a reversal of the usual downstream-fining trend in coarse channel sediments.

1. Introduction

The effect of floods on channel morphology and
sediment transport has been a subject of continuing
inlerest since the pioneering studies of Wolman and
Miller (1960) and Wolman and Gerson (1978), These
studies noted the imponance of floods on channel mor-
phology in basins with high flow variability or resistant
channel boundaries. Subsequent studies have
attempted to "quantify these relations (Gupta. 1975.
1988; Baker, 1984; Baker and Pickup, 1987; Miller,
1990; Wahl, 1992). This paper examines the effecI of
spatially disjunct rainfall-induced flash flooding on

1 Present address: floodplain Administrator. City of Ft. Collins.
P.O. Box 580. Fl. Collins, CO 80522. USA.

0169.555X/95/$09,50@ 1995 Elsevier Science B.V. All rights reserved
SSDIO 169.5S5 XO 169.555X (95) 00037.2

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coarse-sediment deposition in a high-gradient channel
of the Colorado Rocky Mounlains.
The Colorado Rocky Mounlains are represenlalive
of many mounlainous areas of the world in thai flood
magnitude-frequency relalions are difficult to estimale
as a result of limited knowledge of flood hydromete-
orology. Rocky Mountain flood hydrometeorology is
poorly understood because of the complex meteoro-
logic condilions in mountainous areas, the sparsily and
shan duralion of precipitation and discharge records,
and the mixed population of floods resulting from either
rain.on-snow. snowmell. or rainfall (Jarrell. 1987.
1990).
. Of the three types of floods in the Colorado Rockies,
intense rainfall floods are the most catastrophic. They
are characterized by a rapid increase in river stage. over

Journal: GEOMOR Article: 368

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