Historical channel analysis and its application to riparian and aquatic habitat restoration

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Abstract
The planning, design and evaluation of a restoration project should be guided largely by an understanding of past channel changes. A historical analysis can sometimes reveal underlying causes of channel change and document prior habitat conditions, both useful in setting appropriate objectives for restoration. Restoration planning should address the historical causes and patterns of channel degradation that cannot be detected by examining current conditions alone. Moreover, ongoing adjustments in the channel and changes in the catchment must be understood when interpreting channel changes following construction of restoration projects. Changes in channel form (and the independent geomorphological variables of run‐off and sediment load from the catchment) can be documented from a variety of sources, including historical maps, boundary lines, aerial photography, bridge and pipeline surveys, gauging records, field evidence and archival sources. Historical riparian vegetation, and use by fish and wildlife, may also be documented from early survey records, photographs and written accounts. Historical analysis should cover an area large enough to capture all events potentially influencing the project reach. The entire catchment upstream should be examined to identify events affecting the flow regime and sediment load, such as deforestation or dam construction. For channels in erodible alluvium, the study should include the channel downstream to the first stable grade control to capture events whose effects may propagate upstream, such as channelization or base lowering. Application of historical channel analysis to the San Luis Rey River in California served as a basis for evaluating the potential for (and hydrological constraints upon) riparian restoration.