SOURCE AND OCCURRENCE OF DEBRIS FLOWS
The more recent debris flows [less than about 5,000 years ago (Miller, 1980)] that are not related to eruptions were caused by heavy precipitation, outburst floods from glacier-, moraine-, or debris-dammed lakes. The frequency and magnitude of these debris flows were affected by the high, steep slopes of erodible pyroclastic flow deposits and presence of earlier debris-flow deposits that resulted from eruptions on Mount Shasta.
Nonvolcanic-caused debris flows generally originate near the termini of glaciers above the 2,740-m altitude, an average 2.4 km from the summit, or downstream in the steep-walled canyons where banks of loose eruptive debris are continually sloughing into the channel (fig. 4 below). Bedded lava topping pyroclastic flows adjacent to active stream channels, which is subject to collapse because of lateral undercutting by stream erosion, is an additional source of material for debris flows.
At least 69 debris flows of noneruptive origin during the last 500 years (before 1985) were identified by Osterkamp and others (1986). The magnitude and frequency of these flows and the procedures used for dating are discussed by Hupp (1984), Osterkamp and others (1986), and Hupp and others (1987). An analysis of debris-flow activity between 1900 and 1985 (table 2) indicates that at least 37 flows occurred on the various streams, with an average interval of 2.3 years between flows. Some debris flows occurred on streams originating from snowfields that are regions of permanent snow cover, such as the upper areas of Diller Canyon (fig. 2). Because the snowfields varied extensively depending on precipitation during the previous winter, the analysis of debris-flow activity for streams downstream from snowfields was separated from those streams downstream from glaciers. The 1900-85 period was selected for detailed analysis because dates of debris flows were more reliable than for earlier flows, and climatic data for the vicinity of Mount Shasta were available.
For those streams originating from glaciers only (table 2), 30 debris flows were observed during 1900-85, giving an average interval of 2.8 years between flows. During any one year, several streams may experience a debris flow as shown below:
Streams affected
Year by debris flow
1935 Whitney, Bolam
1937 Ash, Gravel
1939 Ash, Gravel, Inconstance
1955 Bolam, Mud
1958 Ash, Gravel
1977 Whitney, Mud, Ash
Debris flows occurred most often since 1900 during 1918-39 and 1955-77 (fig. 5A below). Debris flows in the 1930's and 1970's occurred during droughts, whereas the 1950's and early 1960's was a period of above-average precipitation. There appears to be no relation between antecedent conditions and the occurrence of debris flows as shown in figure 5B below, in which the number of years since the previous debris flow on a stream is plotted. The data in table 2 indicate that for 1911-85, Mud Creek was the only stream affected by debris flows in successive years (1924-26). The cause of these successive annual flows on this stream is uncertain.
During 1900-85, more debris flows occurred on Mud and Whitney Creeks (fig. 6 below) than on any other streams on Mount Shasta. The number of debris flows recorded on a given stream, however, is related to the location of the reach studied because large-magnitude flows travel farther downstream than smaller flows. For example, a small debris-flow deposit on Whitney Creek at an altitude of 2,440 m (fig. 7 below) did not reach the debris fan. For this study, most debris flows large enough to extend below timberline (about 2,590 m) were documented. Large debris flows that traveled below an altitude of 2,130 m were observed on Mud, Ash, Gravel, Inconstance, Bolam, and Whitney Creeks. The debris flow on Whitney Creek in July 1985 extended downstream to an altitude of 914 m.
