Role of bedrock ground water in the initiation of debris flows and sustained post-flow stream discharge

The Wasatch Front near Salt Lake City, Utah was the site of numerous debris flow events during the springs of 1983 and 1984. These flows were related to the rapid melting of an above average snow pack. These flows originated as localized failures of colluvial sediment on the upper slopes of the Wasatch Front which had developed on a Precambrian metamorphic bedrock. Following the debris-flow event, "normally dry streams" continued to flow well into the autumn. These sustained flows suggest drainage of a ground-water reservoir, "tapped" by the slope failure and scour of the channel. The ground water contained within the colluvium could not sustain the observed stream flow at the canyon mouth. Therefore, we believe that a bedrock reservoir provided the source of water for sustained stream discharges. Field observations and mapping indicate that the metamorphic bedrock has been highly fractured. Erosion control and range management practices following the 1930 debris-flow events combined with a gradual recovery of the Salt Lake area moisture balance effectively recharged this bedrock aquifer. Excess recharge during the rapid spring melt overcharged the bedrock aquifer, increased the pore-water pressure in the colluvium at locations of bedrock springs and seeps, and led to localized slope failures which mobilized the down canyon colluvium into debris flows. Drainage of the bedrock aquifer, at exposed springs and seeps, maintains the sustained stream discharge seen at the canyon mouths.--Modified journal abstract.