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

Influences on Peak Flows

Changes in landcover can influence the size of peak flows in rivers and streams. For example, rates of water infiltration and storage vary due to loss of vegetation by fire or by timber harvest. Further, infiltration rates also decrease as permeability of the land surface decreases. Decreased infiltration and storage of water causes streams and rivers to respond more rapidly to rainfall events. Rapid response to rainfall is referred to as "flashiness". Flashiness increases the frequency and severity of peak flows in the river and its tributaries.

Stream response to rainfall becomes more rapid as land surfaces in the watershed become less permeable. Forest fire can reduce soil permeability temporarily. Removal of vegetation increases runoff until regrowth occurs. More permanent decreases in permeability occur due to road building and soil compaction. Road systems act as secondary drainage networks, further increasing the speed of runoff and the size of peak flows. For example, runoff amount and the rate of stream response in Knopti Creek, Siskiyou Fork, Hurdygurdy Creek, Jones Creek, Goose Creek, and Rock Creek have increased following timber harvest (McCain et al. 1995) presumably due to reduced vegetation and areas of reduced permeability such as compacted skid trails.

The tendency for high peak flows can increase due to changes in snow hydrology. In clear cuts, snow accumulates in larger quantities and melts more rapidly (McCain et al. 1995) which tends to increase peak flows. These characteristics increase the magnitude of "rain-on-snow" events. During these events, snow at mid-elevations (about 3000-5000 feet) melts rapidly with the onset of warm rain. In this situation when a large snow pack melts rapidly, the result can be devastating peak flows.

Peak flows are also influenced by riparian vegetation on the flood plain. As flood waters flow through riparian vegetation, water velocity decreases. Reduction in water velocity effectively stores floodwater and therefore reduces peak flows downstream. On the lower Smith River, construction of levees has greatly modified the interaction between peak flows, riparian vegetation, and the flood plain.

Increased frequency and severity of peak flows has many repercussions. Recovery intervals are decreased and the likelihood of inner gorge landsliding is increased. There is more frequent removal of riparian vegetation and redistribution of woody debris.

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