Relative Effects on a Low-Volume Road System of Landslides Resulting from Episodic Storms in Northern Idaho

Abstract

In late November to early December 1995 and February 1996, northern Idaho was hit by heavy rains on a deep snowpack, resulting in two flood and landslide events of historic magnitude. Each of these storms was larger than the previous significant storm, which occurred in January 1974. A study was initiated by the U.S. Department of Agriculture Forest Service to survey and study the effects of the resultant landslides on the Clearwater National Forest, including the effects on the aquatic ecosystem. The results of this study were compared with the estimated average natural sediment resulting from landslides to evaluate the incremental impacts of these recent episodic landslides. They were also compared with the results of a study conducted on the landslides resulting from the January 1974 storm to determine if the landscape was responding more severely to large storms as a result of Forest Service management activities over the past 21 years. The general results of this study indicate that, of the Forest Service management activities, roads are the major contributor; however, they contribute less sediment than natural landslides. The total resultant sediment appears to be within the transport capacity of the aquatic system, and the landslide response in 1974 was similar to the 1995–1996 response. The results of the aquatic ecosystem study were generally mixed, with some habitat parameters indicating degradation, some unchanged, and some improved as a result of the flooding or flooding with landslide sediment.

Postfire erosional processes in the Pacific Northwest and Rocky Mountain regions

“…The difference in occurrence should depend on the relative probability of debris slides and the relative frequency of overlandflow generation. However, even in areas with known high debris slide hazard, there is great variation among geologic parent materials and among landforms in the relative susceptibility of slopes to debris sliding, regardless of the degree of disturbance (Swanson and Dyrness, 1975;McClelland et al, 1999). Similarly, the likelihood of surface runoff is dependent on climatic, soil and vegetation factors, and both soil properties and vegetation change with time following fire.…”

Sediment concentration and turbidity changes during culvert removals

“…During a 25-year return period rain-on-snow event on the Clearwater National Forest, there were 526 landslide events that occurred on roads similar to those obliterated at Wendover Creek and Horse Creek (McClelland et al, 1999). Their data implied an average of 400 000 kg of sediment were delivered to the stream from each road related landslide.…”

The roles of natural and human disturbances in forest soil erosion.

“…Watershed scale observations will tend to reflect decade to century trends in erosion rates, with large sedimentation events associated with infrequent watershed disturbances or flood events (e.g. McClelland et al, 1997;Kirchner et al, 2001). Both managers and the public tend to focus on erosion and sediment delivery occurring in the first year or two following a disturbance.…”