FAILURE OF THE HOLOCENE-AGE LAKE KSNEA DAM

Almost 8,000 years ago, a catastrophic landslide deposited over forty-million cubic meters of debris into the Skagit River near Marblemount, Washington, creating the seventeen-kilometer-long Lake Ksnea . Just over 1,000 years later, the cataclysmic eruption of Mt. Mazama deposited sixty-six million cubic meters of volcanic tephra within the Skagit River watershed upstream of the lake. Over a period of one year, an estimated sixty-one million cubic meters of tephra was eroded and hydraulically transported to Lake Ksnea, where the tephra suspension settled in layers of up to seventeen meters thick. Approximately one hundred and fifty years later, the Lake Ksnea dam disappeared, leaving only stratigraphic records within the valley walls.

Results indicate that a combination of factors caused the destruction of the dam. The presence of steep valley walls and low-permeability tephra both acted to deter normal water flow; including groundwater flow. Numerous tectonic events also acted to destabilize the extremely sturdy dam, which was composed of angular schist and relatively stable sediments and tree logs. Results indicate that a large earthquake just west of the dam occurred 6,400 years ago during a very wet spring snowmelt, resulting in a landslide that deposited into Lake Ksnea. The resulting wave of water topped the dam, causing internal erosion, and eventual failure. Additionally, due to the massive amounts of low-permeability tephra on the bottom of Lake Ksnea, it was calculated that the tephra only discharged 1% of incoming water from the lake, even though the tephra accounted for 11% of total post-landslide lake volume. It is therefore believed that the tephra deposits caused the dam to fail because of reduced lake holding capacity, low permeability, as well as the shear volume of the tephra on the lake floor. Springtime slope saturation, natural weakening of the dam, and the continuous barrage of water resulting from the aforementioned landslide into the lake all interacted to cause the destruction of the dam 6,400 years ago.