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

Riparian Influences

Composition and width of the riparian vegetation community differs markedly between the narrow bedrock controlled channels of the upper watershed and the wide alluvial channels on the lower river. Bedrock-controlled channels in the upper watershed generally have narrow riparian strips of water demanding species such as willows and alders. In downstream riparian areas where slopes are gentler, these species, and cottonwood, are more abundant. In the alluvial channels, riparian vegetation changes along a gradient from the channel margin to the higher terraces (Table 26). The active channel is scoured annually and therefore supports little vegetation. Only annual herbaceous plants and willows are able to grow in the active channel.

 

Table 26. Successional sequence of vegetation in riparian areas.

1

Sparse herbaceous vegetation

2

Low density early-successional scrub

3

Dense mid-successional scrub

4

Riparian forests

5

Drought tolerant forests on upper level terraces.

 

As the height of the terrace increases relative to the river surface, scouring and inundation by peak flows occurs less frequently. Because disturbances are less frequent on the higher terraces, vegetation succession can move forward. Vegetation on the flood plain is commonly modified during high flows as large volumes of water rip through riparian forests, leaving linear stands of forest interspersed by younger scrub vegetation.

Riparian vegetation influences stream habitat in many ways. Removal of the riparian canopy increases the solar energy input to the stream thereby increasing water temperatures. For example, the California Department of Fish and Game attributes high water temperatures in certain streams to upslope logging. Summer water temperatures reached 71 ° F in Jones Creek in 1977 and 1978 following heavy logging. On Hurdygurdy Creek, also heavily logged, water temperatures in the same period rose to 74 ° F. (California Department of Fish and Game 1980).

Reduction of the riparian canopy along the tributaries can be significant even when temperatures remain tolerable in the adjacent stream. Maintenance of water temperatures farther downstream in the mainstems depends on cool water supplied by the tributaries. Small temperature increases in many tributaries can have significant cumulative effects on water temperatures downstream.

Riparian vegetation encourages deposition of fine sediment on the flood plain during peak flows. As flood waters move through the riparian vegetation, they decrease in velocity. As water velocity is reduced, the water has less capacity to carry sediments. Sediments are deposited which improves the soil and nourishes riparian vegetation on the flood plain. In addition, sediment inputs to downstream areas are reduced.

Riparian vegetation influences the stream ecosystem in many additional ways. The riparian area provides the stream ecosystem with nutrients from insects, leaves, and branches. At high elevations, the riparian vegetation may prevent or reduce ice formation. When willow logs are buried during a flood, the willow bushes that sprout up create habitat complexity and increase roughness in the channel. By affecting water velocity and creating roughness in the channel, riparian vegetation also plays a complex role in channel evolution.

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