0423-1201-SDTDC: Riparian Restoration, Page 6

Riparian Restoration

CHAPTER 1: ECOLOGICAL INTEGRITY (CONTINUED)

Structure and Function (Continued)

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).

Photo of tree and brush vegetation along a stream bank.
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 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).

Photo of large woody debris (LWD) lying in a stream.
Figure 15—LWD in a stream.

Photo of large woody debris (LWD) in a larger river.
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.

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. Even-aged stands reflect sporadic natural flooding events that periodically remove trees and shrubs.

Photo of a sharp river bend, bordered by dense vegetation.
Figure 17—Flood plain.

“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).