«DRAFT ENVIRONMENTAL ASSESSMENT ENVIRONMENTAL ASSESSMENT FOR THE COLUMBIA STOCK RANCH SECTION 536 ECOSYSTEM RESTORATION PROJECT The environmental ...»
Additionally, the SEF allows for background considerations of metals in sediments and the Evarts, R.C. 2002, Geologic Map of the Deer Island Quadrangle, Columbia County, Oregon, and Cowlitz County, Washington, U.S. Geologic Survey, Miscellaneous Field Studies Map MF-2392.
Screening levels were developed by the interagency Regional Sediment Evaluation Team (chaired by the Corps, Northwestern Division, and EPA-Region 10) for use in the SEF.
1 mg/kg = 1 part per million April 22, 2016 Page 26 reported arsenic concentration falls within the range of background soil concentrations measured for the Coast Range region of Oregon (ODEQ 2013).
In addition to sediment quality, subsurface explorations were conducted between July and October 2015 to facilitate evaluation and design considerations for construction of the setback levee and seepage berms. Hand augers, borings and cone penotrometer testing was conducted to evaluate sub-surface conditions on the CSR project site. Holes were drilled to depths generally between 30 and 50 feet, with the deepest cone penotrometer test reaching a depth of 100 feet below the ground surface. Pervious sand with silty layers was encountered at depths 11 to 12 feet below silty alluvial material at the surface. At the time of drilling, groundwater was encountered between 3.5 and 7.5 feet (NAVD 88). The silty alluvial material ranged from 2 to 3 feet near the Deer Island Slough pump station, 5 to 10 feet thick near the center of the CSR project site.
Environmental Consequences No Action Alternative Under the No Action Alternative, the geology, topography, and soil type and structure of the CSR project site would remain unchanged from existing conditions. Short-term construction related impacts, including the opportunity for increased erosion, compaction, and importation of non-native material would not occur. Current levees, dikes, and drainage channels would remain in place and continue to limit tidal influence on the CSR project site, providing limited benefit to physical processes dependent on tidal exchange and hydrologic circulation of the lower Columbia River. Water control structures would remain in place and continue to limit or restrict alluvial sediment transport processes and erosion/accretion rates. No off-channel habitat would be protected or restored, and floodplain wetlands would be disconnected from the mainstem river and channel forming processes would be absent from the CSR project site.
Proposed Action Construction of the setback levee, modifying the existing levee, and excavating tidal channels and marsh habitat throughout the CSR project site would impact local geology, soils, and topography than other project components. These effects include temporary erosion and sedimentation, altered channel form, structure and density, localized changes in velocity, flow and circulatory patterns influencing channel form and function, and increased groundwater exchange resulting in changes to soil structure and porosity. Short-term construction-related impacts include temporary increase in soil erosion, compaction, and mixing of soil horizons associated with staging and access roads, earthwork and grubbing of vegetation.
Following implementation of the Proposed Action, soil erosion would occur during and immediately after levee breaching, when destabilized soils would be exposed to daily tides and high flows. However, soil erosion would decrease rapidly over time as the site becomes stabilized and vegetation becomes established on the project site. Because vegetation communities in the lower river are highly dependent on elevation and inundation frequency, as tidal marsh habitat develops and habitat succession occurs, site topography and elevation could change in response to sediment accretion and localized patterns of erosion that occur as a function of increased tidal exchange and the restoration of alluvial and sediment transport processes. Accretion is expected to balance with erosional forces over time to establish a selfsustaining marsh ecosystem.
Soils in the project area have slightly elevated arsenic concentrations, but concentrations are within range of natural background concentrations found in the region. For this reason, there April 22, 2016 Page 27 would be no adverse impacts from disturbing these sediments during construction and the Proposed Action does not present a substantial ecological risk. In addition, sediment quality represented by the Corps’ sampling framework are suitable for unconfined, aquatic placement and exposure.
The Corps conducted a geotechnical analysis to evaluate the range of impacts to physical characteristics of the CSR project site following implementation of the Proposed Action. The analysis modeled seepage, internal erosion, stability, groundwater observations and estimates of settlement. Observations of geologic conditions following historic flooding events (for example, the 1948 Columbia River flood) reveal that numerous sand boils have occurred on the CSR project site, indicating that the setback levee may be susceptible to failure via internal erosion of the levee foundation. Internal erosion occurs when water flows through a cavity in an embankment, washing fine particles of soil out of the core ultimately leading to failure of the embankment. Analysis results indicate the thin alluvial cover and pervious sand underlying the alluvium is susceptible to heave and internal erosion at the design height of the setback levee (32 feet NAVD 88), but inclusion of seepage berms on the landward side of the setback levee would alleviate this potential issue. Further, during the 1948 flood, the existing levee with the relatively steep side slopes was stable and the proposed setback levee is assumed to remain stable given its flatter design and similar foundation conditions.
4.3. Vegetation, Wetlands and Aquatic Habitats Historic topography sheets (t-sheets) indicate this portion of the Columbia River floodplain was largely composed of wetland forest and freshwater, emergent marsh along the floodplain channels. Wetlands on the project site are a mosaic of riverine, palustrine, and lacustrine (including forested and emergent vegetation communities and permanent, semi-permanent, seasonal, and temporary hydrologic regimes). Riverine, forested wetlands are found bordering the Columbia River north of the levee. This area is seasonally flooded and consists of a mosaic of deciduous vegetation and other woody strata. Many native plants are crowded out by a dense monoculture of RCG found throughout the CSR project site, degrading overall habitat condition.
Following construction of the Columbia River Levee in 1942, arable lands south and west of the levee were managed as pasture for cattle grazing and are now largely dominated by non-native grasses. Aquatic habitats south of the levee are a continuum of palustrine (forested and emergent) wetlands and open water ponds. These wetland areas provide habitat for an assortment of waterfowl, amphibians, and isoloated fish populations, but are disconnected from regular hydrologic exchange with the mainstem Columbia River. Forested areas buffering the wetland complex west of Tide Creek, where elevations are slightly elevated, also support seasonally flooded forests. Tide Creek bisects the property north-to-south and is bordered by a riparian corridor, providing the only stream habitat on the CSR project site.
In July 2015, the Corps conducted a wetland delination to evaluate the status of wetlands on the CSR project site. Emergent, forested, shrub, and tidal wetlands were documented primarily along the waterways and lowland areas of the site (see Figure 8). Emergent wetlands are common and the herbaceous-dominated lowlands include fringe wetlands along waterways, depressions, and sloped wetlands. The total acreage of emergent wetlands on the site is approximately 117.4 acres, ranging between 11.5 and 14 feet (NAVD 88). Hydrology in the area is impounded by the railroad and levee and the emergent wetlands riverward of the levee are connected to tidal wetlands and channels which are directly influenced by the Columbia River. Forested wetlands are located in the southern portion of the site, totaling approximately
1.5 acres. Shrub wetlands are uncommon and located in the central portion of the site, totaling
April 22, 2016 Page 29 Common vegetation communities on site range from low diversity areas of pastures, fence rows, and hay fields to more diverse areas of riparian forests, ponds, emergent wetlands, and tidal wetlands. Dominant forest species include big-leaf maple (Acer macrophyllum), black cottonwood (Populus balsamifera), black hawthorn (Crataegus douglasii), Oregon ash (Fraxinus latifolia), nootka rose, elderberry (Sambucus racemosa), Himalayan blackberry (Rubus armeniacus), and red alder (Alnus rubra). Dominant pasture species include velvet grass, red fescue (Festuca rubra), perennial rye grass (Lolium perenne), barnyard grass (Echinochloa crusgalli), bluegrass (Poa palustris), orchard grass (Dactylis glomerata), and RCG. Wetland plant species include wapato, arrowhead (Sagittaria longiloba), large-leaf pondweed (Potamogeton natans), common velvetgrass, and water purslane (Ludwigia palustris). Other common plant species across the site included cat’s ear (Hypochaeris radicata), tufted hairgrass, Canadian thistle (Cirsium arvense), Pacific willow, Douglas fir (Pseudotsuga menziesii), soft rush, white clover (Trifolium repens), elderberry (Sambucus racemosa), and bentgrass (Agrostis scabra).
Environmental Consequences No Action Alternative Under the No Action Alternative, vegetation throughout the CSR project would continue to provide low quality habitat to fish and wildlife. Invasive species would not be sufficiently managed and would likely continue to spread throughout the project area. Native vegetation would continue to be limited by non-native species.
Wetlands are complex ecosystems that perform a variety of important physical, chemical and
biological functions which are essential to the health of the environment, including:
Water storage and flow regulation: Wetlands provide temporary storage of storm flows, which reduces erosion and flood peaks, as well as maintenance and recharge of water flows during dry periods.
Water quality protection and improvement: Wetlands retain and filter excess nutrients, sediments, and contaminants, and native vegetation communities support water quality through temperature moderation and shade cover.
Biological productivity: Wetlands provide diverse habitats for fish and wildlife, including breeding grounds, nesting and foraging sites, refuge, and other critical habitat for a variety of fish and wildlife.
The extent of wetlands in the CSR project area would remain the same and functional processes would be limited under the No Action Alternative. Construction-related impacts to wetlands would not occur and wetland functions and values would not improve, limiting water storage, flow regulation, and filtration, and provide no benefits to fish and wildlife.
Proposed Action Hydrology would be the driving force determining the composition of plant communities and wetland function following implementation of the Proposed Action. The duration and frequency of tidal and seasonal inundation would return to a more natural hydrologic regime.
These factors are expected to increase the abundance and diversity of native wetland plants and reduce the composition and spatial extent of non-native species over time. Low elevation marsh habitats would experience daily tidal inundation throughout most of the year, and planting dense concentrations of native wetland species in the excavated marshes would reduce the likelihood of the RCG dominating the aquatic habitats. Higher elevation marsh habitats would be planted with a combination of native “wet” tolerant herbaceous plants, April 22, 2016 Page 30 shrubs and trees. The plant community on the CSR project site is expected to undergo successional changes, largely seeding itself from surrounding sources over the long-term and develop into shrub-scrub habitat.
Two primary emergent wetland areas (14.6 acres and 10.3 acres) would be affected by construction activities associated with implementing the Proposed Action at the CSR project site. Excavation of tidal marsh habitats and construction of the setback levee would result in the temporary loss of approximately 20 acres of emergent, tidal, and forested wetlands.
However, the restoration of hydrologic connectivity is expected to result in a substantial net gain to the quality and quantity of wetlands in the project area. In response to the Proposed Action, the establishment and restoration of wetlands in the project site is expected to directly result in increased water storage capacity and flow moderation during flood events, improved water quality via retention and filtration, and indirectly support overall biological productivity by providing feeding and rearing habitat and nesting, and foraging opportunities, benefiting a wide variety of fish and wildlife.
Following implementation of the Proposed Action, the CSR project site would be dominated by three main habitat types: tidal channels and streams, marsh, scrub shrub and forested wetlands. Riparian and upland forests would be maintained to the maximum extent practicable, but some trees would be removed during construction of the setback levee. It is important to note that planned vegetation communities would not adhere to a prescribed planting plan. Rather, a diversity of native species would be planted at variable densities at species-specific elevations to promote varied vertical and horizontal strtucture across the project site according to the different project elements.
Construction would require the use of scrapers, graders, excavators, dump trucks, and other heavy equipment as needed. Equipment would be used to create intertidal channels, marsh habitats, and scrub-shrub habitat on the floodplain. Trees removed during construction of the setback levee would be used during the restoration activities associated with tidal channels and marsh habitats to simulate beaver activity and fallen trees from adjacent riparian areas or drift wood from the Columbia River. Because the CSR floodplain would be a low-energy environment, trees would not be anchored using mechanical means (rebar), but would be free to move with the tides and high flood waters.