R iparian zones are the interfaces between terrestrial and aquatic ecosystems. As ecotones, they encompass sharp gradients of environmental factors, ecological processes, and plant communities. Riparian zones are not easily delineated but are comprised of mosaics of landforms, communities, and environments within the larger landscape. We propose a conceptual model of riparian tones that integrates the physical processes that shape valleyfloor landscapes, the succession of terrestrial plant communities on these geomorphic surfaces, the formation of habitat, and the production of nutritional resources for aquatic ecosys-Riparian zones have been investiing a diverse and often confusing array of definitions based on hydrologic, topographic, edaphic, and vegetative criteria (see reviews in Karr and Schlosser 1978, Swanson et al. 1982). Most riparian classification system focus on a few selected at-tems.

Coarse woody debris from streamside forests plays important biological and physical roles in stream ecosystems. The distance from stream bank to rooting site was determined for at least 30 fallen trees at each study site on 39 streams in the Cascade and Coast ranges of Oregon and Washington. The study sites varied in channel size (first- through third-order), side-slope steepness (3 to 40°), and age of surrounding forest (mature or old-growth stands). The distribution of distances from rooting site to bank was similar among streams, with 11% of the total number of debris pieces originating within 1 m of the channel and over 70% originating within 20 m. Stands with taller trees (old-growth conifers) contributed coarse woody debris to streams from greater distances than did stands with shorter (mature) trees.

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Riverscapes are complex, landscape-scale mosaics of connected river and stream habitats embedded in diverse ecological and socioeconomic settings. Social-ecological interactions among stakeholders often complicate natural-resource conservation and management of riverscapes. The management challenges posed by the conservation and restoration of wild salmonid populations in the Columbia River Basin (CRB) of western North America are one such example. Because of their ecological, cultural, and socioeconomic importance, salmonids present a complex management landscape due to interacting environmental factors (eg climate change, invasive species) as well as socioeconomic and political factors (eg dams, hatcheries, land-use change, transboundary agreements). Many of the problems in the CRB can be linked to social-ecological interactions occurring within integrated ecological, human-social, and regional-climatic spheres. Future management and conservation of salmonid populations therefore depends on how well the issues are understood and whether they can be resolved through effective communication and collaboration among ecologists, social scientists, stakeholders, and policy makers. I n the early 1800s, Lewis and Clark described an ecologically diverse and biologically productive riverscape in the Pacific Northwest -the Columbia River Basin (CRB): "The multitude of this fish (salmon), indeed, are almost inconceivable" (Lewis et al. 1814). As EuroAmerican explorers and settlers began moving westward at the turn of the 19th century, so too did the destruction of these riverscapes. The construction of 56 major hydroelectric dams on numerous mainstem rivers within the CRB since the 1930s has fragmented this riverscape and severed connections between critical habitats of migratory salmonids (salmon, trout, and char). Extensive urbanization along rivers and floodplains, along with the widespread release of hatchery-raised fish, have also contributed to the decline and extirpation of several salmon and steelhead (Oncorhynchus mykiss) stocks, resulting in the listing of several species and distinct populations under the US Endangered Species Act (ESA). Other human activities, such as overfishing, pollution, agriculture, grazing of rangelands, logging, railroads, diking and draining of river floodplains, beaver (Castor canadensis) eradication, mining, and the introduction of non-native species, have also had detrimental impacts on the CRB (Lichatowich 2001;Rieman et al. 2015).At present, the CRB is one of the most heavily managed river basins in the world. Bernhardt et al. (2005) estimated that, since 1990, more than $1 billion has been spent annually on river restoration throughout the US with many projects focusing on degraded riverscapes in the Pacific Northwest, including the CRB. For example, the Northwest Power and Conservation Council's fish and wildlife program cost $782 million in 2014 and $757 million in 2015, which included funds for the protection of migrating fish through investments in improving fish passage and ...

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