FACTORS ASSOCIATED WITH OCCURRENCE OF DEBRIS FLOWS
The occurrence of debris flows have been recorded during the summer season only. This indicates that warm air temperatures, and ablation in association with antecedent glacial movement and precipitation, are primary factors associated with the formation and passage of debris flows on the flanks of Mount Shasta. Specific conditions causing debris flows are generally a combination of water released from impoundments, snow and glacial melt, and direct precipitation on the various stream basins during the summer snow/glacial melt season.
Release of Water from Channels Blocked by Debris Deposits
Lava flows that lie unconformably on the land surface or debris deposits that are highly fractured periodically break off, falling into a channel and causing the potential for temporary blockage of the channel (fig. 8 below). This debris then becomes a source of material for transport by future debris flows. The potential for channel blockage is also illustrated at sites on Mud and Whitney Creeks (fig. 9 below), where local scour and bank failure formed caverns in the loose, uncompacted material that subsequently collapsed and created temporary dams. These dams cause lateral channel migration and create the potential for flow impoundment. The sudden release of water from the impoundment, sediment transported from upstream reaches, plus entrained material in the debris dam could cause the formation of debris flows that travel to lower reaches of the channel.
The influence of ground water is another factor in the formation of debris deposits. Ground water recharged by precipitation or snowmelt, and perched on less permeable eruptive deposits and seeping from the pyroclastic deposits, may cause slump failure and mass movement downslope into a stream channel. These deposits may block the channel and create a small lake that incorporates sediment and flows downstream when the dam is breached (Marron and Laudon, 1987).
Release of Water from Channels Blocked by Snow and Ice
The only detailed account of debris-flow formation due to channel blockage by snow and ice was provided by Williams (1934). A number of observers, however, have reported that ice flows and flood flows on Mud Creek have lodged in the canyon as a result of ice or debris blockage. Flood flows also may undermine the canyon walls, resulting in landslips (slides) that form temporary dams that may be breached, causing debris (mud) flows (Denand, 1924; Williams, 1934). The eyewitness accounts in the following paragraph provide the best evidence of the relation between channel blockage by the breakup of glacial snow and ice, formation of temporary impoundments during flood flows, and the subsequent occurrence of debris flows.
In his description of the debris flows during 1924-31, Williams (1934) indicated that a large torrent of water emerged from under the glacier, which he described as "tumbling down the cliff below, and carrying with it large blocks of ice from the glacier front, the swollen stream rushes down the deep and narrow canyon, undermining its loose banks of tuff and breccia. From time to time parts of the steeper eastern wall of the Mud Creek canyon collapse into the gorge, forming temporary dams behind which the water is impounded until able to overflow and carry away the obstacle. The onrushing flood (debris flow) then continued down the canyon for about 10 km, spilled over its banks, and left behind in the flat country (debris fan) thick sheets of bouldery detritus (fig. 3) in a matrix of sand and mud that crossed the McCloud River Railroad and inundated the road bed (fig. 10 below).
Although no debris flows threatened the town of McCloud during 1925 (Hill and Egenhoff, 1976), it is considered probable that small debris flows caused by the meltwater described by Egenhoff in 1925 occurred on Mud Creek. Wood (1931) indicated some debris-flow deposits occurred on the fan of Mud Creek and in the McCloud River in 1925.
Snow bridges (fig. 11 below) are formed by the scouring action of streamflow as meltwater from more exposed parts of the mountain flows downstream. Most snow bridges are formed in deep-shaded parts of incised channels on the mountain. When these bridges catch earth and silt (debris) as described by the Mount Shasta Herald (July 21, 1977), or the snow tunnels collapse, a temporary reservoir may be formed. These dams then may be dislocated or breached, thereby creating surges of water and debris that may cause debris flows downstream.
Debris flows caused by glacier outbursts and snowmelt (Williams, 1934; Hill and Egenhoff, 1976) generally have occurred during the warm summer months (Meier, 1969; Miller, 1980; Blodgett and others, 1988). Debris flows attributed to the effects of warm air temperatures have occurred on Mount Shasta a number of times. The relation between air temperature and glacial melt is not one of cause and effect; rather, air temperature is an index of solar radiation that actually causes melting of snow and glacial ice (Tangborn, 1980), which in turn combines with material/sediments beneath and down gradient from glaciers, to initiate debris flows. Hill and Egenhoff (1976) and Wood (1931) reported that debris flows were seen on Mud Creek in 1881 and 1920 and repeatedly during the summer months of 1924, 1925, 1926, and 1931. The most common occurrences were in July (1926) and August (1924, 1925, and 1926). The Redding Searchlite and the Mount Shasta Herald reported that tons of muddy water flowed down Mud Creek during July 1977. On July 6, 1985, a debris flow reportedly caused by glacial meltwater occurred on Whitney Creek.
Direct Runoff from Precipitation
Heavy or intense rainfalls on the snowfields and glaciers of Mount Shasta have been noted over the years (Nutting, 1935; Carter, 1987). Convective precipitation in the vicinity of the mountain is especially common during June, July, and August. On August 28, 1935, a deluge of water, mud, sand, and boulders that flowed down Whitney and Bolam Creeks (fig. 2) first covered and then washed out the Southern Pacific tracks. Studies of the Whitney, Bolam, and Graham Creek basins by Southern Pacific indicated that the resulting debris flows were due solely to rainfall (Southern Pacific Transportation Co., 1935). However, as shown in table 2, no debris flows that were documented in this report from the 1935 storm actually occurred on Graham Creek.
The August 1935 debris flow on Whitney Creek continued downstream 6 km, blocking U.S. Highway 97 (fig. 2) and partly burying several automobiles (fig. 12 below). In a report of this flood by the California Department of Highways and Public Works, Nutting (1935) reported that the debris flow was attributed to a "warm thunderstorm on the [Whitney and Bolam] glaciers that started the snow and ice to melt and the resulting flow soon assumed the proportions of an avalanche." The assumption that the August 1935 debris flow on Whitney Creek was caused by precipitation is confirmed by a review of climatological data reports for 1935 (Bowie, 1935). Although climatic data for the storm on August 28, 1935, are not available for the Whitney Creek area, data from the U.S. Department of Agriculture Weather Bureau climatic stations at nearby Yreka and Montague indicate that a storm front passed through the area August 26-29, 1935, and that precipitation was greatest on August 28 (Bowie, 1935). The dates of miscellaneous phenomena summary, as given in the climatological data report for the month of August, gives the 28th as the date of a thunderstorm at Montague, 45 km northwest of Whitney Creek (Bowie, 1935). The debris flows on Whitney and Bolam Creeks on August 28, 1935, therefore, are attributed to intense convective precipitation and melting snow centered on parts of the Whitney and Bolam Creek basins.
