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Initiation and Frequency of Debris Flows in Grand Canyon, Arizona

SETTING

Grand Canyon has formed where the Colorado River cuts deeply through the southwestern corner of the Colorado Plateau in northern Arizona (fig. 1), exposing nearly 2 km of Paleozoic and Proterozoic stratigraphy (fig. 3). The combination of the slow downcutting of the Colorado and the gradual rise of strata toward the Kaibab uplift in the west results in the rapid exposure of Paleozoic strata as one moves downstream (Huntoon and others, 1986). Numerous resistant strata—the Paleozoic Kaibab Formation, Coconino Sandstone, and especially the thick Redwall and Muav limestones—are exposed at river level, resulting in a narrow, steep-sided canyon. Owing to the steepness of the canyon walls, the divides for most drainages in Marble Canyon are at the rim, exposing the maximum extent of the stratigraphy. Marble Canyon encompasses much of eastern Grand Canyon.

See fig 3.
Figure 3. Stratigraphic column showing rocks exposed in Grand Canyon and the distances in river miles downstream from Lees Ferry where they first appear along the Colorado River (from Billingsley and Elston, 1989).

 

The entire Paleozoic section and some Proterozoic units are exposed west of Phantom Ranch (river mile 87; fig. 1). The exposure of the Bright Angel Shale near river level results in a distinctly wider canyon. The drainage divides of many smaller tributaries are not at the rim; therefore, these tributaries do not contain some of the younger geologic units. The maximum dip in the regional structure is mostly to the southwest (Huntoon and others, 1986). From eastern to western Grand Canyon, increased faulting results in large changes in the elevation of stratigraphic units from one rim to the other (Huntoon and others, 1986). Western Grand Canyon lies entirely within Grand Canyon proper.

 

Elevations in Grand Canyon range from 975 to 2,804 m above sea level at the rim, and from 939 m to 402 m along the river. The river itself drops an average of 1.5 m in every linear kilometer. The climate is semiarid to arid, producing a wide range of annual and seasonal precipitation (table 1). Melis and others (1994) and Webb and others (1996) discuss the regional hydroclimatology in relation to debris-flow initiation. Precipitation generally increases with elevation, and the amount of summer precipitation generally decreases towards the west.

 

Table 1. Characteristics of climate stations in the vicinity of Grand Canyon National Park

Station Name1 Elevation
(m)
Record
Length
Mean Annual
Precipitation
(mm)
Summer
Precipitation
(%)
Winter
Precipitation
(%)

Bright Angel RS 2,726 7/48-3/95 646 29 60
Desert View 2,271 9/60-7/952 347 40 48
Grand Canyon 2,204 10/04-3/95 403 42 46
Lees Ferry 978 4/16-3/95 148 50 38
Mount Trumbull 1,818 10/20-12/783 297 49 37
Peach Springs 1,613 7/48-3/95 280 45 43
Phantom Ranch 834 8/66-3/95 234 39 49
Tuweep RS 1,551 7/48-12/864 306 42 43

Notes:
1 All stations are in Arizona (Fig. 1).
2 Daily data from September 1, 1960, to July 1, 1975, have been lost at this station, which is not part of the NOAA network of climate stations. Monthly data is available after September 1960 from the National Park Service.
3 Station discontinued.
4In 1986, Tuweep Ranger Station was discontinued as a cooperative observer station, which records rainfall in 0.01 in. accuracy and reports in increments of daily rainfall. A tipping-bucket recording rain gage, which records rainfall in 0.10 in. increments and reports hourly as well as daily rainfall (e.g., U.S. Department of Commerce, 1966), remains in operation.

 


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