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 Smith River Project > Cycles of Disturbance and Recovery

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

Cycles of Disturbance and Recovery

Disturbances have pervasive influences on ecosystem integrity. In the Smith River network and throughout the Pacific Northwest, high quality stream habitats cannot exist without destructive episodes caused by disturbances, such as heavy scouring during high flows. Because disturbances such as fire, floods, and timber harvest occur with different intensity in different areas, effects on stream habitat also vary across the landscape. Different parts of the river system will be in different stages of recovery, and the location of high quality stream habitat will change over time.

Following episodes of increased mass wasting and sediment loads, a recovery period usually occurs. The distinction between disturbance and recovery is blurred because of the time lag between sediment input in upstream areas and the total effect on habitat downstream. For example, in the years following a major disturbance such as the 1964 flood, some areas of the river system may be recovering from the impacts while other areas may be incurring delayed impacts as sediment loads arrive from upstream.

An important recovery process is the flushing of fine sediments from the system. Due to their small size, fine sediment particles are transported out of the system more readily. Therefore, barring further sediment inputs, the average particle size of sediment in the stream will gradually increase. This generally increases primary productivity, benthic macroinvertebrates, and fish populations.

Following upheaval in the stream, woody debris quantities in streams may change. When a peak flow event redistributes large woody debris in the stream, large wood is usually buried in the stream channel. At the beginning of a recovery period, there may be little woody debris in the stream channel. However, as the recovery period continues, the channel is incised and uncovers buried woody debris thereby increasing stream habitat complexity. During recovery, riparian vegetation becomes reestablished near the stream channel, which stabilizes stored sediment somewhat and creates future sources of woody debris.

Timing of a disturbance greatly influences its impact on the system. For example, in California, summer drought does not interfere with normal ecosystem functioning. However a drought during the winter can have many effects on stream ecology such as reduced anadromous salmonid production. For another example, a stream may recover in ten or fifteen years from a storm having a recurrence interval of 100 years. However, if the same storm occurs a few years after a severe forest fire, there may be rates of mass wasting that have not occurred for a thousand years. The stream may be impacted for many decades.

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