Here and there along the Truckee River you can find huge boulders made of granitic rock, which originated long ago in the Sierras and were later carried downstream by the river. Last week I saw one such boulder out near Ambrose Park, which reminded me of the crazy story of how these gigantic rocks may have ended up so far from the mountains.
Back in the 1960s, a researcher named Peter Birkeland investigated four major glaciations (periods when temperatures were cooler and glaciers were advancing) that took place in the Tahoe-Truckee region during the Pleistocene epoch, approximately 2.5 million to 11,700 years ago. These were called the Hobart, the Donner Lake, the Tahoe and the Tioga glaciations (his paper is here).
During the Tahoe glaciation (the third of the four glaciations), a glacier in Squaw Creek is believed to have descended into the Truckee River Canyon, forming an ice-dam that blocked the flow of the Truckee River. Behind the ice dam, the level in Lake Tahoe began to rise, eventually reaching 90 feet above its modern-day water level.
Studies on glacier-dammed lakes around the world show that as water rise behind an ice-dam, the ice eventually begins to float, releasing huge floods of water from underneath the glacier. In Iceland, this kind of flood is called a jokulhlaup.
In a second study, Birkeland calculated the depth and velocity of the “Jokulhlaup” that would have resulted from the breakup of the 90-foot ice dam at Lake Tahoe. Just below the dam, Birkeland calculated that the water would have been moving approximately 30 feet per second. Near Verdi, the floodwaters would have been 40 to 80 feet deep, and 40 feet deep out at Mustang. A flood of this size, says Birkeland, would have been plenty big enough to move even the largest boulder, which is located in Verdi and measures approximately 40 x 20 x 10 feet.
According to Birkeland’s study, “Tahoe outwash at Mustang contains boulders up to 4 feet across that were transported across Truckee Meadows, and many larger boulders were derived from the canyon east of Truckee Meadows.”
Over the weekend, I went for a hike out at Mustang Ranch Preserve, The Nature Conservancy Preserve located on the lower Truckee River east of Reno. I was looking for some fall colors and some big boulders, both of which I found. Not being much of a geologist, I can’t tell you for sure if the boulders arrived at Mustang via ice-aged floods or some other means, but they were boulders and they were very large. What do you think?