Background The 90.5-acre Vanport Wetlands site is located west of Interstate 5 and south of the Expo Center in north Portland. This mitigation site was purchased to compensate, or mitigate, for wetland impacts at several locations on Port property, primarily for aviation purposes. Historically, the Vanport Wetlands site was part of the Columbia River floodplain; however precipitation and storm water runoff are now the sole sources of surface water. Flooding is controlled on the site by a pump station, which is managed by the Multnomah County Drainage District (MCDD). At the time of purchase, approximately 62 acres of the 90.5-acre property were existing wetlands dominated by reed canarygrass.
Mitigation Plan The goal of the mitigation plan is to increase diversity of plant and wildlife habitat on the site by establishing a native plant-dominated wetland habitat with a surrounding vegetative buffer. The mitigation plan was designed to alter the site's hydrology by capturing precipitation during the rainy season and modifying the existing pumping/drainage regime to allow increased water depth within the wetland basin. This creates an open water habitat in the winter and spring months. The extended duration of inundation is used to stress and ultimately reduce the reed canarygrass so that native wetland species can become established. The design also incorporated a number of physical modifications to the site, including a low, earthen berm in the northern portion of the property in order to prevent the flooding of Expo Road and the site’s north ditch and reconfiguration of the drainage channels to provide a more meandering swale system. The plan included plowing and reseeding the wetland, enhancement of the upland habitat and enhancing the vegetated buffer around the boundary of the property.
Status Physical modifications to the site were completed in December 2001, and planting was completed in March 2003. In October 2001 the site was renamed Vanport Wetlands, and in November 2001 it won the Columbia Slough Watershed Council's Achievement Award. In 2004, the Port won the Oregon State Land Board Award for the Vanport Wetlands project.
Site modification, planting and increased flooding have resulted in approximately 65.5 acres of wetland habitat in the original central wetland. All of this acreage has now been used to meet the Port’s mitigation obligations from a number of projects. Monitoring of site conditions to compare to performance criteria will be ongoing through at least 2010. Weed control, primarily for reed canarygrass, thistle, teasel, blackberry and nightshade, has been ongoing since the property was purchased in 1999.
Vanport Wetlands provides habitat for over 100 species of birds, many of which nest and raise young on the site during the breeding season. In 2005 the Port confirmed that yellow-headed blackbirds had successfully nested and fledged young – Vanport Wetlands may be the furthest west this species is known to breed in Oregon. Many mammals have been observed including small rodents, beaver, muskrat, coyote and black-tailed deer. Amphibian species that are frequently observed on the site include the Pacific treefrog, long-toed salamander and bullfrog; and, reptiles include the common garter snake and western painted turtle.