RIPARIAN RESTORATION
This chapter suggests a broad interpretation of the riparian ecosystem and its influences. Often riparian ecosystems are described as land and vegetated areas associated with lakes, streams, rivers, and wetlands that have complex ecosystems and provide food, lodging, and travel corridors for both aquatic and terrestrial species. It is not limited to delineating riparian ecosystems as being 50 or 100 feet from a river’s edge. Readers are challenged to develop an understanding of the structural and functional characteristics of this type of ecosystem and their key roles in maintaining ecological integrity. See figure 6.
Figure 6—This drawing illustrates how each factor impacts and influences the others, thereby strongly influencing riparian ecosystem health (Ward 1975).
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ECOLOGICAL INTEGRITY
The influences on the riparian ecosystem should be viewed topographically from upstream to downstream, from upslope to downslope, and from subsurface up through the vegetated canopy. This concept is better expressed as the longitudinal, lateral, vertical, and temporal influences. See figure 7.
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The longitudinal influence extends the length of the stream.
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The lateral influence begins in the water body and extends through the riparian vegetation, into the upland forest or dry land vegetation, to the point where overland flow (runoff) is initiated.
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The vertical influence extends below the dryseason water table and through the canopy of mature vegetation. In certain settings, grass may be the naturally mature vegetation; in others, it may be cacti or scrubby trees.
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Temporal influences are changes to the riparian ecosystem over time. Riparian ecosystems are not tied to a specific distance from the water’s edge but rather to changes in vegetation type, soil type and moisture availability, and other ecological characteristics. Structural and functional characteristics combine to create ecological integrity for the riparian landscape. If the structure or function is compromised, the consequences will be apparent in the degradation of the area. See figures 8 and 9.
Figure 7—Identifying the riparian ecosystem.
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RIPARIAN RESTORATION
Figure 8—Riparian ecosystem cross section 1.
Figure 9—Riparian ecosystem cross section 2.
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ECOLOGICAL INTEGRITY
Riparian ecosystems provide the most important link between upland forests and aquatic habitats and have a unique array of functions. See figures 10 and 11.
runoff and overland flow, providing storage and infiltration areas. See figure 13.
Figure 10—The area that drains into a lake is made up of one or more watersheds. The riparian ecosystem that surrounds a lake protects and promotes water quality, aquatic ecosystem health, and shore stability.
Figure 12—In this example, alders form a canopy over the stream.
Figure 11—Typical mixed hardwood and conifer riparian ecosystem.
Canopies of large trees (structure) mesh to create a microclimate that functions to cool the riparian ecosystem, lower water temperatures, and shelter wet areas. In the absence of trees, low-growing plants and grasses provide shade as do undercut banks. See figure 12. Canopies of riparian forests produce particulate organic matter, primarily leaf litter, which is an important energy base for replenishing soil nutrients and aquatic food webs.
Palik and others (2000) found that one-third of leaf litter in streams originates from more than 30 meters away. The plant structure (standing and dead and down) of riparian vegetation, leaf litter, and uneven ground captures sediment and slows and detains
Figure 13—Live and dead plant materials contribute to structure and function.
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RIPARIAN RESTORATION
Plants with flexible stems and rhizomatous root systems lie flat against the ground as floodwaters wash over them, shielding the ground from erosion.
These plants, upright shrubs, and tree species also slow waterflow and encourage sediment deposition.
Root systems and soil organisms create an interwoven structure that holds soils together, stabilizing streambanks. See figure 14. They catch and hold pollutants, use phosphates adhering to the soil and sediment particles deposited by runoff or floodwater, and hold harmful or toxic substances in place by minimizing soil movement (Ministry 1994).
revetments that shelter the shore, causing beaches to form.
LWD is generated from several sources. See figures 15 and 16. Murphy and Koski (1989) suggest that as much as 50 percent of LWD originates from within 1 meter of the stream. In another study, Murphy indicates that a 30-meter, uncut riparian buffer zone is needed to maintain long-term LWD input (Murphy
1995). In mature and old growth forests in Oregon and Washington, for example, LWD is generated from within 20 meters of the stream (Dolloff 1994).
Figure 15—LWD in a stream.
Figure 14—Healthy soil is essential for plant growth and vice versa.
Litter and standing biomass are also necessary for soil regeneration. Without the cycle of litter and biodegrading, flooding, and plant growth, soil would not be replenished. Without soil, plants cannot grow; without plants, the soil is washed away. When soil is impaired or the topsoil is threatened, plants are less plentiful, have smaller basal areas, and have more space between them (Griggs and Stanley 2000). Such poor vegetation cover leaves the soil even more exposed to raindrop impact, sheet runoff, and erosion. As little or no moisture is held in the ground, the water table drops, water quality suffers, and fewer plants survive. Stream surface flows may decrease or cease.
Large woody debris (LWD) affects the configuration of a stream by diverting waterflow and forming pools.
Debris helps regulate storage of sediment and particulate organic matter, and provides aquatic habitat
(Murphy 1995). LWD also affects lakeshores by providing aquatic habitat and by forming natural
Figure 16—LWD in a river.
In arid climates, woody debris may be carried from nearby mountains or be generated locally by native shrubs and sporadic stands of trees such as cottonwood or green ash. Debris catches on and piles up against rocks or outcrops.
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Flood plains, another important part of riparian ecosystems, trap sediment and particulate organic matter outside the active channel during overbank flows (Swanson and others 1982; Harmon and others.
1986, as seen in Gregory and others 1991). See figure
17. Riparian ecosystems regulate not only the movement of water between terrestrial and aquatic ecosystems, but also the movement of nutrients, sediments, and particulate organic matter over flood plains.
Riparian vegetation on active flood plains is created by, adapted to, and rejuvenated by flooding. Evenaged stands reflect sporadic natural flooding events that periodically remove trees and shrubs.
“Non-flood plain forests [the riparian ecosystem on terraces and slopes beyond the active flood plain and surrounding lakes and open-water wetlands] trap sediment in overland flows before they enter the aquatic system. Tree boles, coarse woody debris, herbaceous vegetation, and litter all trap sediment in overland flow” (Palik and others 1998).
Figure 17—Flood plain.
ECOLOGICAL INTEGRITY
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