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Baseline Monitoring Study of Restoration Effectiveness in the Green River (Mile 32): Process and Habitats in the Channel and Floodplain

Baseline Monitoring Study of Restoration Effectiveness in the Green River (Mile 32): Process and Habitats in the Channel and Floodplain

The goal of this study was to characterize baseline conditions at restoration sites on the Green River to support future evaluations of the physical effectiveness of levee setback and removal.

Abstract

The study area encompasses three projects near river mile 32: Auburn Narrows and the Fenster and Pautzke sites from Project MG-18 of the WRIA 9 Salmon Habitat Plan. Explicit hypotheses were proposed in a Before-After design, and linked to quantitative metrics including: (1) channel processes; (2) riparian processes; and (3) aquatic habitat. Metrics were evaluated with a combination of historical photo analysis and field surveys. Conditions were quantified in the pre-levee and flow regulation (baseline) period and compared with conditions in the post-levee and flow regulation (pre-restoration) period. This comparison demonstrates that the river pattern has been converted from a dynamic, multi-thread system to a stable, single thread channel.

Lateral migration slowed by approximately 60%, facilitating the expansion of the floodplain forests dominated by willow and cottonwood. Pioneering vegetation covered 45% of the existing bars. Pioneer bars presently contain approximately 190 cottonwood and 12,000 willow stems per hectare, indicating tree regeneration is occurring. Several Douglas-firs established on rocky bars, but naturally-regenerated conifers are otherwise absent. Forests eroded more slowly, with declines ranging from 45 to 81%, depending on patch type. Reductions in bank erosion contributed to a 52% decline in the average recruitment rate of large logs, which provide the essential habitat function of logjam initiation. In spite of this decline, wood loading is presently high (361 pieces km-1), owing to a recent chute cutoff. Aquatic habitat analysis demonstrated that, from the baseline to the pre-restoration period, the mainstem channel narrowed substantially (e.g., from 77 to 39 m) and became more homogeneous. Side channels increased in length though backwater channels retracted. Bar edge and terrace bank edge habitat declined by 47% and 42%, respectively, owing to the expansion of hardened banks and floodplain bank edges.

In summary, the channel and riparian processes and aquatic habitat in the study area have been substantially altered from baseline conditions. In the post-restoration period, levee setback and removal may be considered ecologically successful if monitoring hypotheses are confirmed and the river begins to exhibit a shifting mosaic steady state. In such a state, conditions (e.g., the composition of the landscape and biological communities) and processes (e.g., rates of bank erosion, cutoffs, vegetation establishment) fluctuate over time but are relatively predictable and increasingly stable over the long term. The specific distribution, sequencing, and extent of the changes are somewhat uncertain. This uncertainty is in part, the result of the underlying dynamics of a functioning river ecosystem.

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Green River Restoration Monitoring Report (667 Kb)

Appendix A  (646 Kb)

Appendix B  (111 Kb)

Appendix C1  (4 Mb)

Appendix C2  (4 Mb)

Appendix C3  (2.2 Mb)

Appendix D  (164 Mb)

Appendix E  (56 Mb)

Appendix F  (93 Mb)

Appendix G  (25 Mb)

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