Debris Flows Near Buena Vista, Colorado

By Jeffery A. Coe, Jonathan W. Godt, and Rex L. Baum

WARNING: Provisional Report, Subject to Revision

A thunderstorm on the evening of Monday, July 22, 2002 caused debris flows in the canyons of Cottonwood Creek and Chalk Creek, west and southwest of Buena Vista, Colorado (fig. 1). All of the flows initiated on hillslopes underlain by decomposed granitic rocks. Along Cottonwood Creek, debris flows crossed Chaffee County Road 306 in eleven places and trapped several motorists (fig. 2). Joe Nelson of Chaffee County Road and Bridge Department (verbal commun. 2002) estimated that 20,000 m3 of mud was deposited on and near County Road 306 (fig. 3 and fig. 4).

The debris flows temporarily blocked or diverted Cottonwood Creek in four places (fig. 5). Many smaller debris flows did not reach the road. The same storm triggered a debris flow from the Chalk Cliffs that blocked County Road 162 about 1.5 km southwest of Mount Princeton Hot Springs (fig. 6 and fig. 7). A few smaller debris flows appear to have occurred on the Chalk Cliffs, farther to the west, but none reached County Road 162.

Heavy rain started about 6:00 p.m. A county employee (Clayton Ogden, verbal commun. 2002) and an area resident (Col. George Meaders, retired, verbal commun. 2002) both reported that the debris flows began to occur within 30 minutes after rain started falling. The Red Deer RAWS rain gauge, 2-3 km north of Cottonwood Creek (fig. 2) recorded 0.87 inches (22 mm) of rain between 6:00 and 7:00 p.m., with an additional 0.18 in. (5 mm) between 7:00 and 8:00 p.m.

Field observations made on Thursday, July 25 above Cottonwood Creek indicate that the debris flows started as a result of erosion, rather than starting as small landslides. At some locations we observed evidence that the debris was eroded from the head of alluvial or talus fan deposits where storm water flowed from a bare rock channel onto the head of the fan (fig. 8). At other locations, we observed either single or coalescing rills at the head of a freshly incised channel that appeared to be the main source of the debris (fig. 9). For example, at a small debris flow above Cottonwood Creek, we observed an incised channel near the fan apex with feeder rills above it (fig. 10).

The debris-flow deposits along both Cottonwood Creek and Chalk Creek were composed of abundant fine gravel and coarse sand along with angular granite clasts and a few boulders of about 1 m in the longest dimension. Debris was deposited in both Cottonwood and Chalk Creeks and in a few places small, temporary ponds were formed.

Figure 1 

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Figure 1 Location map

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Figure 2 

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Figure 2 Area along County Road 306 affected by debris flows.

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Figure 3 

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Figure 3 Remnant of debris-flow deposit that blocked Chaffee County Road 306.

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Figure 4 

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Figure 4 Remnant of debris-flow deposit that blocked Chaffee County Road 306

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Figure 5 

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Figure 5 Debris-flow deposit that diverted Cottonwood Creek.

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Figure 6 

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Figure 6 Remnant of debris-flow deposit that blocked Chaffee County Road 162 below the Chalk Cliffs.

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Figure 7 

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Figure 7 Debris-flow deposits from the July 22 event and erosion of older deposits adjacent to Chalk Creek and downslope from Chaffee County Road 162.

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Figure 8 

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Figure 8 Eroded fan head at the mouth of an exposed bedrock channel (background). Erosion entrained enough sediment to convert a water flow into a debris flow (foreground).

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Figure 9 

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Figure 9 Debris flow source area and deposit above Cottonwood Creek.

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Figure 10 

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Figure 10 Close-up view of debris flow source area showing rills and incised channel.

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