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Pool Spacing, Channel Morphology, and the Restoration of Tidal Forested Wetlands of the Columbia River, U.S.A.

Authors

  • Heida L. Diefenderfer,

    Corresponding author
    1. Pacific Northwest National Laboratory, Marine Sciences Laboratory, Sequim, WA 98382, U.S.A
    2. College of Forest Resources, University of Washington, Seattle, WA 98195-2100, U.S.A
      Address correspondence to H. L. Diefenderfer, email heida.diefenderfer@pnl.gov
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  • David R. Montgomery

    1. Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, Seattle, WA 98195-1310, U.S.A.
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Address correspondence to H. L. Diefenderfer, email heida.diefenderfer@pnl.gov

Abstract

Tidal forested wetlands have sustained substantial areal losses associated with human land use, and restoration practitioners lack descriptions of ecosystem structures within these systems, in which surface water is a significant controlling factor on flora and fauna. In particular, the roles of large wood in tidal areas remain poorly described compared to terrestrial and riverine ecosystems. This study documents the role of large wood accumulations in forcing channel morphology in remnant Sitka spruce (Picea sitchensis)–dominated tidal freshwater wetlands (swamps) in the floodplain of the Columbia River, U.S.A., near the Pacific coast. The average pool spacing documented in channel surveys of three swamps near Grays Bay, 2.2–2.8 channel widths per pool, was not significantly different. There were higher numbers of pools on these tidal forested wetland channels (median 2.7 pools/100 m) than on a nearby diked agricultural site prior to restoration. Log jams were common in the swamps and nonexistent in the pasturelands prior to restoration. On the basis of pool spacing and observed sequences of log jams and pools, tidal forested wetland channels were classified consistent with a forced step-pool channel type. This new classification for tidal systems provides a basis for restoration project design involving placement of large wood and development of pool habitats for aquatic species. Modifications by beaver (Castor canadensis) observed on restoration and reference sites warrant further investigation to explore the interactions between these animals and restoration methods that affect channel structure and hydraulics in tidal forced step-pool channels.

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