Landslides to Logjams

Introduction:

Not all landslides are bad. In fact, the native salmon, steelhead, and trout of Oregon’s North Coast depend on infrequent, periodic landslides on forested slopes. These natural landslides carry trees and other large woody debris into the river, which create and maintain complex instream habitat. These logjams protect juvenile fish from predators, offer shade during the summer when water temperatures spike, provide critical spawning habitat by trapping gravel, and maintain floodplain connection.

An estimated 50 percent of instream large wood comes from landslides on forested slopes. Heavily forested watersheds grow trees up to 500 or even 1000 years old. Western Oregon, Washington, and California salmon streams are built on this architecture of large woody debris. Unfortunately, a clearcut landslide dumps massive amounts of sediment and, unsurprisingly, very little wood into rivers. Landslides on clearcut slopes are much larger and more frequent than slides on forested slopes. In fact, studies show that landslides occur up to 24 times more frequently after an area has been logged. Moreover, the sediment loads dumped into streams by a clearcut landslide often channelize rivers, impair water quality and either wipe-out or bury existing logjams.

Landslides on clearcuts destroy spawning habitat by smothering spawning gravel with fine sediment. Sedimentation also reduces the available rearing space for juvenile salmonids by burying cobble and filling rearing pools. When sediment fills pools and creates broader, shallower channels, it deprives salmonids of vital overwintering habitat, disrupts feeding and rearing, and exacerbates stream temperature problems.

Background:

Goals:

  • Improve habitat complexity for listed Species such as Oregon Coast Coho
  • Protect landslide prone slopes from clearcut logging
  • Improve water quality on 303(d) listed streams


Key Documents:

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