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WWRC 86-05a
Characterizing Riparian Zones

Prior to 1977, the term "riparian" meant little to the general public. Although water judges and fisheries biologists were familiar with the environmental features described by the word riparian, there were many who secretly believed riparians were cousins of the shy little artesians who flavored a Northwest beer. Public awareness of what is "riparian zone" really was and its role in the human environment began with a series of regional and national conferences in 1977 and 1978. Interest and involvement in riparian protection and rehabilitation has grown steadily with additional conferences in 1980, 1981, and 1984. Concern with riparian issues has grown to the point that in July, 1985, the Bureau of Land Management published a draft Riparian Management policy for public lands under its jurisdiction. The effectiveness of this and any other policy depends upon the knowledge and experience of both those who formulate the policy and those of us who review the drafts. Basic to our understanding of how to manage and protect riparian zones is the knowledge of those characteristics which create this unique landscape feature.

If you were asked to define or describe a riparian zone, what would you say? Lawyers and administrators may describe it as that portion of a stream or river channel which carries water during all or part of the year, an engineer or geohydrologist may define it in tarns of flood events and groundwater recharge patterns while a fisheries biologist may discuss the type of streamside vegetation and the shape of banks. Even though it would be more convenient to select one of these definitions for common use, a riparian zone is all of these things and more.

Riparian zones probably support the most complex natural communities in the Intermountain West Both the number of plant and animal species and the intricate interdependence of living and nonliving components of the community give rise to this complexity. Streamflow duration, wildlife populations and the type of vegetation growing along streambanks are very obvious components of the riparian zone, but the whole ecological community is greater than the sum of these parts. Trout, deer, beaver, songbirds, and livestock depend upon the diverse and abundant vegetation of the riparian zone for all or part of their existence. Although some animal species, such as beaver, may modify the plant community, it is the local hydrologic cycle which shapes the character and structure of the riparian zone. The hydrologic cycle is. in turn, formed by the local and regional climate, geologic structure and soils. Examination of how a riparian zone is formed and develops can illustrate the role of the hydrologic cycle, geology, and soils in creating the character or appearance of a riparian ecosystem.

A drainage will pass through four general phases (Fig. 1) as it progresses to equilibrium or harmony with the physical environment

Stage 1. Flow is intermittent or seasonal and usually destructive, heavily scouring the channel bed and walls. Soil is poorly developed and the vegetation community is dominated by annual or short-lived perennial plants.

Stage II. Channel and bank erosion has been reduced and sediment deposition provides the basic material for soil development Soil formation is enhanced by increased plant cover. The presence of more vegetation protects banks by slowing streamflow, thereby decreasing its erosive power. Reduced water velocity allows sediment to settle out further improving soil development

Stage III. As soil development continues, streamflow gradually becomes perennial rather than seasonal. The vegetative community is developing rapidly with more individual plants and more plant species occupying the banks and immediately adjacent areas. The additional plant cover slows runoff in the channel and overland flow from the uplands which increases sediment deposition and begins the formation of floodplains or terraces. Rather than being straight, the channel has begun to meander, further reducing water velocity and decreasing the destructive potential of runoff events. Because of diverse and abundant vegetation and perennial flow, wildlife numbers increase, and livestock grazing levels are increased.

Stage IV. A meandering stream flowing with deep, well-developed soils provides living space for a diverse plant and animal community. Although extreme runoff events may still occur, damage is minimized because of well-sodded banks and the numerous woody species which reduce streamflow velocity.

The rate at which a riparian zone develops is dependent upon the precipitation pattern generated by the climate and the erodibility of local geologic formations. If precipitation is abundant and occurs regularly and geologic formations weather easily, a riparian zone and its attendant ecosystem will form in a relatively short geologic time span. However, if precipitation is scanty or occurs as irregular, high intensity storms or if the local geologic formations weather slowly, development can take a long time. Consequently, the riparian zone is created by the interplay between past and present climatic and geologic processes and the ensuing interaction between colonizing plants and stream channel dynamics. The resulting plant community and streamflow regime are an expression of these conditions and can be used to characterize the riparian zone. As climatic patterns and geology change across the landscape, the character of riparian zones also changes. However, in historic time this general relationship has been altered.

Grazing, mining, logging, farming, highway construction, urban sprawl and recreation can alter streamflow and streambank vegetation whether the use occurs in the riparian zone or adjacent uplands. The alterations tend to disturb the dynamic balance of climate, streamflow, and vegetation and usher in a series of changes which ultimately change the character of the riparian zone. If the disturbance is repeated or continues for a long period of time, the resulting character of the vegetation/stream system may bear little resemblance to the potential ecological community which formed under the existing climatic and geologic conditions.

Riparian zones negatively influenced by human settlement and resource use will go through the following phases of retrogression: (1) accelerated streambank and channel erosion; (2) increased instream sediment loads; (3) loss of resident trout and insects; (4) loss of certain vegetation species or classes; (5) increased incidence of destructive floods; (6) further loss of banks and vegetation; and (7) loss of perennial flow. Those zones formed in areas where geologic weathering is slow or precipitation scanty are more resilient to abuse while those formed in areas with high precipitation or very erodible geologic material will degrade quite rapidly. But, wherever the zone is on the scale from stable to degraded, it will still have characteristics which reflect the local, surface hydrology.

In general, a riparian zone is marked by the presence of a channel, the duration of flow which occurs in that channel, and the plant community which can survive on the amount of water available in the banks. A zone with seasonal flows will have little soil development. Because soil holds the water plants need for survival, only drought tolerant perennials or annuals will occupy the banks. If streamflow is perennial and a deep soil exists along the channel, water is available for a longer period. Drought sensitive plant species can survive on this site and will either protect the riparian zone or enhance its development. Plant species or community types are reliable indicators of streamflow duration and soil development. Because wildlife and humans are dependent upon the availability of water, the presence water, the presence and abundance of animal life will be greater in well-developed riparian zones. But wildlife, especially tenestrial species, are only indicators of riparian zone health because they rely on the plant community. Eliminate a wildlife species and it can be reintroduced and become established again. Degrade the riparian zone and many species will not survive even if reintroduced because the plants they depend on will be absent We are no different. Without a stable, well-developed riparian zone there will be few resources (forage, irrigation water, timber) to use, and those that might exist will be too few or stunted to support many people.

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