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 Smith River Project > Landsliding and Mass Wasting

Chapter 4 -- Watershed Processes and Aquatic Ecosystems (continued)

Landsliding and Mass Wasting

Sediment is often delivered to streams by various types of landslides which are collectively termed mass wasting". Except for the gentle coastal plain, most of the Smith River Basin is inherently landslide prone (California Department of Fish and Game 1980). Areas composed of weak rock, such as the Galice and Franciscan formations, are especially subject to erosion, mass wasting, and sediment production.

Mass wasting is more likely when soils have high water content. During heavy storms the pores in the soil become filled with water, increasing the possibility of activating or reactivated landslides. Areas with greater local relief have increased pore pressure of water in the slopes which increases the tendency for mass wasting. Therefore, when other factors are equal, higher rates of mass wasting occur on steeper slopes. Areas near larger streams tend to have greater local relief and therefore higher rates of mass wasting than tributaries higher in the watershed. Landslides can be triggered by removal of the toe of a slope by flood waters or road building.

Mass wasting also occurs in the form of debris torrents which are essentially wet landslides that move down a stream channel, scouring down to bedrock. The resulting smooth bedrock channel is devoid of habitat complexity and may be virtually unusable by fish. Debris torrents sometimes occur naturally but may also be triggered by human influence. For example, during a storm, a culvert may be blocked by woody debris causing water to back up. This may cause the road crossing to fail suddenly and initiate a debris torrent that washes down the channel. Debris torrents often move large quantities of woody debris downstream.

In January 1970, following rains, a large landslide fell from the northeast face of Rattlesnake Mountain and blocked the South Fork Smith River. Known as the Rattlesnake Slide, it eventually covered nearly a square mile and moved between 1.5 and 2 million cubic yards (California Department of Fish and Game 1980) or as much as 5 million cubic yards (McCain et al. 1995). A contributing factor was road building activity at the toe of the mountain and road construction for logging on the upper slopes. The slide blocked the South Fork for a few hours, creating a lake two miles long and 50 feet deep. When the river breached this blockage, a large wave of water was released. Although there was not memorable damage, flood surges caused by stream-damming landslides are potentially disastrous for downstream areas (Burke 1995, California Department of Fish and Game 1980).

Aerial photographs from the 1940s show relatively few streamside landslides. There were a few slides along the North Fork and Middle Fork (McCain et al. 1995). In the entire Smith River basin, there are now over a thousand landslides including hundreds over 200 feet wide (McCain et al. 1995, California Department of Fish and Game 1980). Recent large slides, such as the Rattlesnake Slide on the South Fork and others on the Middle Fork, will continue to influence channel characteristics and stream gradients, both upstream and downstream, for many decades.

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