Stream restoration has many ecological challenges that are unique to lotic (flowing) ecosystems. Streams are constantly changing environments and can be thought of as being in a state of dynamic equilibrium. This increases the difficulty of predicting how various ecosystem components will react, and can change interactions on relatively short timescales such as after large floods. Unknown or unexpected ecological responses to particular actions can occur despite adequate site research and planning. Also, determining restoration potential can be difficult, because finding nearby reference streams with minimal human impact may be impossible.
Stream restoration occurs at many spatial scales, and scale-associated factors must be considered when designing restoration actions. Stream order, a hierarchical size classification system where streams are grouped in relation to tributary size, is critical as ecological relationships between organisms and their environment can change across stream sizes. In addition, human activities have varying effects on streams of different orders, due to differences in dilution capacity and water/sediment contact time. Since channel size is frequently related to watershed size, stream order provides additional restoration challenges. Larger watersheds regularly contain more sources of pollution and permanent human alteration such as urban development, sewage/industry outfall, cultivated land, and nonpoint pollution runoff.These increased pollution sources reduce the ability to mitigate all negative influences on ecosystem integrity. Permanent watershed alterations also limit the restoration potential of a stream, as their negative impact cannot be completely mitigated. An additional scale issue concerns the size of the restoration project. Whether the project encompasses a pool, reach, segment, watershed, etc., changes the necessary approach (Table 1). Larger projects must often take into consideration tributaries and ground-water input, a greater impact from atmospheric deposition, and problems due to the stream flowing through multiple geographic and political boundaries.
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