Formatted Title:

Habitat and Restoration Considerations for Sediment Cleanup in a Tidally Influenced Freshwater Wetland

Background/Objectives:

In St. Helens, Oregon, historical wastewater discharges and management practices from industrial operations dating back to 1929 have released contaminants and wood wastes to adjacent tidally influenced freshwater wetlands (Site). Intertidal drainage channels flow through the 168‑acre Site to Scappoose Bay, which is connected to and influenced by the lower Columbia River. The Oregon Department of Environmental Quality is conducting an Interim Removal Action Measure (IRAM) at the Site to remove sediments and process wastes that pose a significant risk to human health and the environment. The removal area is within a thriving riparian and palustrine wetland that provides habitat to culturally significant native species (e.g., salmon, lamprey, wapato, and tule). A complex network of more than 16,400 linear feet of intertidal channels connects the lowlands and facilitates ingress and egress of native fishes. These channels provide important rearing habitat for juvenile salmon that volitionally move between the various micro-habitats as they forage. In addition, large stands of Oregon ash and black cottonwood provide nesting habitat for avian species. The IRAM and subsequent restoration will consider how to best protect large trees, maintain fish access, expand habitats for native plants and animals, and discourage the spread and re-establishment of noxious and invasive plants while still meeting sediment cleanup objectives.

Approach/Activities:

A baseline habitat and ecological evaluation was conducted at the Site to map and quantify (1) the extent of channels, (2) the distribution of vegetative communities, and (3) the locations and number of large trees (≥ 20-inch diameter at breast height [dbh]). The depths and widths of channel features, channel interconnectivity, and elevation are being used to guide the design of a post-removal channel network whose size and connectivity will equal or exceed the existing network. The spatial extents of vegetative communities were mapped, and the elevations at which the communities occurred were analyzed and compared with the estimated removal depths of the IRAM. The post-removal elevations that would be most beneficial for native plants while decreasing the habitat area for noxious and invasive species (e.g., reed canary grass) were determined. Finally, large trees were mapped, and their approximate crown sizes were used to estimate the extent of their root protection zones. These protection zones will be used to refine removal boundaries such that these valuable native trees will be protected from the IRAM to the extent practicable.

Results/Lessons Learned:

Six major vegetation communities were identified at the Site: inundated emergent, saturated emergent, blackberry shrub, shrub-scrub, ash forest, and lowland forest. Of these, inundated emergent, saturated emergent, and shrub-scrub communities exist within the removal area. Wapato and tule were found in the inundated emergent community, which exists on the borders of large channel features and in mudflats with soft sediments, from 6 to 9.5 feet North American Vertical Datum of 1988 (NAVD 88). Wapato/tule habitat from 8.1 to 9.5 feet NAVD 88 will be increased or maintained and the total area of channel features between 6.0 and 8.1 feet NAVD 88 will be increased or maintained. Reed canary grass was found to dominate the saturated emergent community elevations between 9.5 and 10.3 feet NAVD 88. These elevations will be lowered to expand shallow water habitats and favor the growth of wapato and tule. Ten large trees with 20 inches dbh will be removed to facilitate the IRAM; however, 37 similar large trees will be protected. The large trees removed during IRAM implementation will be re-used as large woody material in the restoration of the Site. The intertidal channel network will be re-established and channel connectivity to Scappoose Bay will be maintained. Together, these actions are intended to maintain or increase important native habitats for years to come.

Joe Krieter

Affiliation

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(Group 2, Poster Board #77) Habitat and Restoration Considerations for Sediment Cleanup in a Tidally Influenced Freshwater Wetland