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<br />Effects. of rainstorms on water and sediment runoff following the 1996 wildfire, Buffalo
<br />Creek, Colorado
<br />
<br />R.D, Jarrett, U,S, Geological Survey, Lakewood, Colorado
<br />T,W, Browning, Colorado Water Conservation Board, Denver, Colorado
<br />
<br />Abstract A basin-wide, monitoring approach provides important information to assess the
<br />effects of wildfire on water and sediment runoff, assess the effects of watershed-rehabilitation
<br />activities, determine watershed recovery time, and help manage forest ecosystems, The
<br />approach is flexible, requires minimal resources, and complements comprehensive instrumented
<br />monitoring in a short amount of time, The approach was applied to the mountain community of
<br />Buffalo Creek, Colorado, which had a catastrophic wildfire on May 18, 1996, Subsequent
<br />rainstorms produced 9 floods larger than a 1 Oo..year (pre-fire conditions) flood as well as
<br />numerous smaller floods, Rates of flood runoff in 1997 were about the same as in 1996, which
<br />indicate persistent effects from the fire and minimal watershed recovery, On July 12, 1996,
<br />about 130 mm of rain fell in an hour and was about a 1,OOO-year event. This extreme rainstorm
<br />produced a flood about 10 times larger than the 100-year (pre-fire) flood, On Sand Draw, and
<br />other small tributaries in burned basins near the center of the storm, peak dischargeswere larger
<br />than the 1,OOO-yr flood, Unburned basins in areas of maximum rainfall had minimal water and
<br />sediment runoff, Study results were used to help the National Weather Service determine
<br />threshold-rainfall amounts that could produce flash flooding in the Buffalo Creek area, Large
<br />quantities of sediment continue to be transported in burned-area streams since the fire, Most of
<br />this sediment is deposited in Strontia Springs Reservoir, which is a major water supply for
<br />Denver located a few km downstream from the burned area, Investigations of alluvial sediments
<br />indicate at least 10 wildfire-flood sequences during about the past 2,500 years in the Buffalo
<br />Creek area, Study results indicate that prehistoric fires and subsequent increased runoff prior to
<br />fire suppression of the last century contribute to cyclical, geomorphic instability,
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<br />Introduction
<br />
<br />"Wildfire is a natural occurrence in this tinder'dry climate, But when humans put themselves in
<br />harms's way, a natural phenomenon can be transformed into a natural disaster,' (Denver Post,
<br />Firefight Starts at Home, p, 4G, July 20,1997),
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<br />Communities are encroaching into forested-mountain areas in many parts of the United States,
<br />particularly at the urban interface near metropolitan areas in the western United States, This
<br />growth has significantly increased the risk of natural hazards to people living in or visiting forested
<br />areas, Floods and debris flows, particularly in recently burned watersheds, pose a serious threat
<br />to human life, property, and the environment. California suffers an average annual billion dollars
<br />in costs and loses to wildfires (Weise and Martin, 1995), Increased risks remain for several years
<br />or decades until burned areas sufficiently recover to pre-bum conditions (Evenstad and Rasely,
<br />1995), Recent wildfires and associated flood and debris-flow hazards in southern California
<br />(Florsheim et ai" 1991; Weise and Martin, 1995); Helena, Montana (Parrett, 1987); Yellowstone
<br />National Pari< (Meyer et aI., 1995; Ewing, 1996); Storm King Mountain, Colorado (Cannon et aI.,
<br />1995); the Wasatch Mountains in Utah (Evenstad and Rasely, 1995); Boise, Idaho; Bandelier
<br />National Monument, New Mexico; and Mesa Verde National Pari<, Colorado (all in 1996)
<br />emphasize the potential risk for loss of life, property damage, and costs associated with hazard
<br />mitigation and watershed rehabilitation, The effects of flood runoff, sediment delivery, geomorphic
<br />changes, watershed recovery time, and the linkages among them are poorly understood for
<br />burned watersheds, Watershed, sediment, and ecosystem models may not be applicable
<br />without modifications for assessing changes due to burned areas (Weise and Martin, 1995), The
<br />focus of this study was to develop a flexible, cost-efficient, monitoring and analysis approach
<br />that can be quickly implemented following wildfires wherever they may occur,
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<br />Research was undertaken to monitor and determine the risk of hazards and to help mitigate, to the
<br />extent possible, loss of life and property damage from water and sediment runoff related to
<br />wildfire areas, Extensive coordination was made with the various government agencies involved
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