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

Synergistic Effects on Stream Habitat

Stream ecosystems respond not only to local disturbances but also to disturbances far upstream. Streams have "downstream impact pathways" so that a major disturbance in a headwater stream can impact all downstream habitats. Thus disturbance events in various areas of the stream network can have cumulative and synergistic effects. Road construction and clear cutting in the headwaters can effect downstream habitats immensely and over long periods of time. There may be a lag period of five to twenty years between a disturbance in the upper watershed and its effects throughout the downstream impact pathway. For example, after logging, five or ten years may elapse before a major storm occurs and triggers landslides in the area. It may be an additional ten years or more before sediment inputs from upstream travel down to important fish-bearing streams. Thus many downstream habitats may be in line to receive impacts that are presently moving downstream or that are likely to be triggered in the future (Frissell et al. 1993). Many downstream areas in the Smith River system may not have felt the full effect of disturbances that occurred over the last twenty years on both federal and private lands. Important ecological refuges may be in jeopardy. If such high risk areas can be identified, preventive restoration efforts, such as storm-proofing roads, would be a wise investment. For discussion of sediment production areas and proposed sediment reduction projects see McCain et al. (1995).

Ecosystem disturbances and interactions often occur as chain reactions and may reinforce each other. For example, higher peak flows increase rates of landsliding, sediment transport, and, in some cases, channel aggradation. Because streams tend to overflow from aggraded channels, the effects of peak flows are further intensified and may undercut nearby slopes. This may cause additional landslides, sediment loads, and aggradation. As peak flows tear out the riparian vegetation there is less resistance to flow and water velocities increase, further heightening peak flows.

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