This document proposes restoration as an everyday management tool for streams whose chemical, physical, and biological habitats have been impaired, a focus that fills the conceptual gap between preservation and remediation. Chapter 1 defines restoration for use by water program managers within the context of the Clean Water Act (CWA) and takes into consideration current understanding of aquatic ecosystems. It is not intended to replace different, and equally valid, definitions that have been offered elsewhere. The restoration of streams fits within a continuum of activities that the U.S. Environmental Protection Agency (EPA) and other environmental organizations have conducted for many years. Activities range from preservation and protection (e.g., the designation and protection of biologically diverse areas as Outstanding National Resource Waters under 40 CFR 131.12[a]) to intense repair/recovery efforts (e.g., highly disturbed areas such as Superfund sites and waters or sediments contaminated with PCBs).
Perspectives on Restoration
Restoration is not solely applicable to severely degraded streams. Although it can be used as an effective tool to return a degraded system to a pre-disturbance condition, restoration is also an important tool for preventing environmental degradation. Strengthening structural and functional elements through restoration can help improve a stream system's tolerance to stressors which lead to environmental degradation.
Restoration has been defined in a number of different ways. On the most basic level, restoration is the process of returning a damaged ecosystem to its condition prior to disturbance (Cairns 1991, Berger 1991, and Caldwell 1991). The long-term goal of restoration is to imitate an earlier natural, self-sustaining ecosystem that is in equilibrium with the surrounding landscape (Berger 1991). A National Research Council report (1992) defines restoration as a holistic process:
Restoration is … the return of an ecosystem to a close approximation of its condition prior to disturbance. In restoration, ecological damage to the resource is repaired. Both the structure and the functions of the ecosystem are recreated … The goal is to emulate a natural, functioning, self-regulating system that is integrated with the ecological landscape in which it occurs.
What does it mean to restore an ecosystem to a prior or pre-disturbance condition? What condition should water resource scientists and managers use as a baseline goal (Westman 1991)? In many cases, restoring an ecosystem to an early pristine or pre-settlement condition would be impossible, because (1) data are insufficient to determine the original condition, (2) species representative of the original condition are extinct, or (3) human activities have changed the soil structure or hydrological characteristics of the ecosystem so extensively that the original condition would no longer be compatible with surrounding ecosystems and landscapes.
Restoration is not yet a perfected approach with accurate and precise predictive capabilities and, in fact, is still "… an exercise in approximation" (Cairns 1991). The practicality and attainability of restoration depend on many factors, including adequate tools (i.e., the state of science and technology), site-specific ecological conditions, social consent, legal authority, and availability of resources (i.e., personnel and funding) (Caldwell 1991). As with other water resource management alternatives, restoration must address questions concerning practicality, predictability of outcomes, and overall effectiveness of specific techniques. Additionally, because ecological systems are complex and may take years to reach equilibrium or fully demonstrate the effects of restoration and other management activities, seeing or measuring results of restoration efforts may take a long time.
Scope of Restoration
Restoration must consider all sources of stress on a stream and is therefore not restricted to instream mitigation of impacts. The health and protection of a waterbody cannot be separated from the watershed ecosystem, and restoration must address all watershed processes that degrade an ecological system, e.g., sediment loading from road cuts or development or increased polluted runoff from impervious areas. The intimate connection of rivers and watersheds is succinctly expressed by Doppelt et al. (1993):
Most people think of rivers simply as water flowing through a channel. This narrow view fails to capture the actual complexity and diversity of riverine systems, and is one of the reasons for failed policies. In the past 15 years many scientific studies and reports have documented that riverine systems are intimately coupled with and created by the characteristics of their catchment basins, or watersheds. The concept of the watershed includes four-dimensional processes that connect the longitudinal (upstream-downstream), lateral (floodplains-upland), and vertical (hyporheic or groundwater zone-stream channel) dimensions, each differing temporally.
Restoration is an integral part of a broad, watershed-based approach for achieving federal, state and local water resource goals. Specifically, restoration is the re-establishment of chemical, physical, and biological components of an aquatic ecosystem that have been compromised by stressors such as point or nonpoint sources of pollution, habitat degradation, hydromodification, and others.
This document emphasizes and endorses the use of natural restoration techniques. Natural techniques that restore a system's ability to approach a pre-disturbance condition are distinct from treatment technologies or structures that are inserted into the system to approximate equilibrium. Natural restoration techniques use materials indigenous to the ecosystem and are linked or incorporated into the dynamics of a river system in an attempt to create conditions in which ecosystem processes can withstand and diminish the impact of stressors.
While this document focuses on the use of natural restoration techniques to achieve water resource objectives, it also recognizes that restoration techniques must in part be selected based on existing landscape conditions. The mitigation of some conditions may necessitate the introduction of structures composed of material not indigenous to the ecosystem to mimic natural processes. For example, the urban landscape surrounding the South Platte River in Denver, Colorado, precluded more natural options for channel reconfiguration and alignment to restore the river's re-aeration potential. Instead, a large concrete drop structure was constructed to ameliorate low dissolved oxygen (DO) conditions. Although this structure uses natural forces associated with the flow of the river, the structure itself would not have developed as a result of natural hydrological processes.
This document recommends a comprehensive watershed perspective for restoration that considers interactions among stressors in developing effective long-term solutions. To facilitate assessment and the development of management strategies, three zones hav e been identified for categorizing stressors and restoration strategies. In actuality, however, the zones below are broadly connected ecologically.
- The instream zone is generally the area that contains the stream's non-peak flows (i.e., the stream or river channel itself).
- The riparian corridor includes the stream channel and also extends some distance out from the water's edge. Odum (1971) provides the following technical definition: Riparian habitats constitute an area of vegetation that exerts a direct biological, physical, and chemical influence on (and are influenced by) an adjacent stream, river, or lake ecosystem, through both above- and below-ground interactions. This area of association extends from the rooting systems and overhanging canopies of streamside flora outward to include all vegetation reliant on the capillary fringe characteristic of soils surrounding aquatic environments. Riparian ecosystems can vary to differences in local topography, stream bottom, soil type, water quality, elevation, climate , and surrounding vegetation.
- The upland zone consists of those areas beyond the riparian corridor within a stream's watershed that generate nonpoint source runoff into the stream and whose infiltration and topographic characteristics control stream hydrology.
Figure 1-1 (not available electronically) provides an example cross section of the three zones: instream, riparian corridor, and upland. Only a fraction of the upland zone is represented in this illustration, because this zone can extend far beyond the stream itself.
Three categories of restoration techniques have been identified for stream management activities that are consistent with the stream zones described above:
- Instream techniques are applied directly in the stream channel (e.g., channel reconfiguration and realignment to restore geometry, meander, sinuosity, substrate composition, structural complexity, re-aeration, or stream bank stability).
- Riparian techniques are applied out of the stream channel in the riparian corridor (e.g., re-establishment of vegetative canopy, increasing width of riparian corridor, or restrictive fencing).
- Upland, or surrounding watershed, techniques are generally related to the control of nonpoint source inputs from the watershed, including hydrologic runoff characteristics from increased imperviousness of the watershed [e.g., urban, agricultural, and forestry best management practices (BMPs)].
Stream restoration can be a mosaic of instream, riparian, and upland techniques, including BMPs, to be used in combination to eliminate or reduce the impact of stressors (both chemical and nonchemical) on aquatic ecosystems and reverse the degradation and loss of ecosystem functions. Instream restoration practices often need to be accompanied by techniques in the riparian area and/or the surrounding watershed. For example, restoration may involve rebuilding the infrastructure of a stream system (e.g., reconfiguration of channel morphology, re-establishment of riffle substrates, re-establishment of riparian vegetation, and stabilization of stream banks, accompanied by control of excess sediment and chemical loadings within the watershed) to achieve and maintain stream integrity.
Balancing and integrating instream, riparian, and surrounding watershed approaches is essential. Any restoration plan could involve a combination of techniques, depending on environmental conditions and stressors to be addressed. Instream and riparian techniques directly restore the integrity of stream habitat, whereas surrounding watershed techniques focus on the elimination or mitigation of sources of stressors that cause the habitat degradation. Because surrounding watershed techniques tend to facilitate a system's ability to restore itself, instream techniques may not always be necessary. In addition, if instream and/or riparian techniques are selected to restore the integrity of the physical habitat, measures that eliminate or mitigate sourc es of stressors that caused the degradation should also be included; otherwise, the restoration effort may fail. Therefore, surrounding watershed techniques should, as a general rule, be considered prior to or in conjunction with the use of instream and riparian techniques. Because many projects need to address both causes and symptoms of stream degradation, combining instream, riparian, and surrounding watershed approaches is often appropriate.
These techniques and specific conditions for their application have been well described in the literature. Table 1-1 provides examples of restoration techniques that fall into these categories and their objectives. Table 1-2 lists selected references containing additional information on these and other techniques.