Extreme winds and windthrow in the western Columbia River Gorge

Severe windthrow in the Bull Run watershed of the Oregon Cascade Range has occurred infrequently over the last 50 yr, yet individual storm events have generated extensive windthrow. In this case study, we consider two storm events known to have caused windthrow in the Bull Run. Using long-term meteorological records, we characterize the events and use the Gumbel extreme value distribution to analyze the wind speeds, estimating return intervals for the two storms. When all annual maximum wind speeds are considered for the 47-yr period of data, the 1973 and 1983 storm wind speeds may occur as frequently as every 5 yr or less, but this increases to a 10-15 yr interval when we analyze only east wind events. Extremely low air temperatures, characteristic of the storms, may have increased the likelihood of windthrow, though on at least one occasion a seemingly similar storm event failed to generate any windthrow. This exercise illustrates the complexity of natural disturbances such as windthrow and the inadequacy of simplistic, meteorologically-based models to predict a stochastic event accurately, particularly when natural patterns of windthrow are regularly altered by forest cutting.