LESSONS LEARNED ABOUT SLOPE STABILITY AND EROSION OF THE FOREST FIRE IN THE COLUMBIA GORGE, OREGON OF 1991: IMPLICATIONS FOR THE EAGLE CREEK FIRE OF 2017

In the late summer of 1991 there was an extensive forest fire in the Columbia Gorge on the Oregon side of the river that was started naturally by lightning. We learned from this fire that this steep terrain underwent three basic erosion/landslide processes in the next ten years as a result of the fire. After the fire was out in the autumn, the first rains brought abundant surface erosion of burnt soil and vegetation. A lot of this ended in the streams. Also, extensive enhanced rock fall occurred in the burned area. One classic area was next to Multnomah Falls where a Brugg cable fence had to be installed to protect the trail leading to Benson Bridge from rock fall onto hikers. Third, we learned that in a period of 5-10 years after the forest fire, areas of intensive burning of the forest would produce very large debris flows. It takes 5-10 years for the roots of the trees burned to disintegrate. Seven large debris flows in 1996 at Dodson and one large one near there in 2001 are examples of this delayed debris flow generation when a “Pineapple Express” would come into the area. This is a rain forest getting over 60 inches of precipitation per year. After the 2017 Gorge the Eagle Creek Fire which also occurred on the Oregon side, but started by two teenagers, we noted the same things. First, there was extensive surface erosion for a week after the first rainfall. Also, all of the roads and trails were closed until checked for rock fall hazards. We now expect debris flows in the next 5-10 years to come down the following drainages that had extreme burning in the headwaters: Tanner Creek, Eagle Creek, Oneota Creek, and Horsetail Creek.