For many decades, farmers built low walls or dikes across coastal wetlands in Oregon to extend their farmland. Now, efforts to restore wetlands and prepare for sea level rise require information about where these dikes are and who is responsible for them.
Since the late 1800s, farmers in Oregon have been building dikes across estuarine wetlands to keep water off the land, increasing the amount of land available to them for growing crops and raising livestock. An unintended effect of the dikes was to reduce marshland habitat used by fish and wildlife populations. Today, sea level rise is threatening to breach the dikes, and rising tides are squeezing remaining natural wetlands out of existence.
Coastal management decision makers, including nonprofit organizations and government agencies that restore wetlands, need a map of the dikes. The location and height of the walls can help them predict how estuarine marshes will respond to various sea level rise scenarios. Knowing the location and condition of all the dikes, as well as who is responsible for maintaining them, is critical for making decisions about which dikes should be removed—to allow wetlands to migrate into shallower water—and which dikes should to be maintained to protect public and private infrastructure.
Staff from the Oregon Coastal Management Program worked with NOAA's Office of Coastal Management to create a geospatial database of dikes and levees in Oregon’s major estuaries. Using lidar, aerial photography, and various map products, they created a draft data layer of dikes and levees, and then classified them according to their condition. Later, the draft maps were verified through fieldwork and participatory mapping methods, working with local experts in each estuary system. Historical records unearthed through the program's efforts revealed the existence of special districts for dike maintenance, so the boundaries of these districts were also included in the maps. The effort also produced a point layer showing tide gates and a polygon layer showing land influenced by levees.
Today, the Oregon Coastal Management Program and the NOAA Office for Coastal Management make the inventory and other geospatial tools available to coastal planners and local governments in a way that is accessible to GIS users with a range of experience levels in the Oregon Coastal Atlas. Groups that conduct wetland restoration work, such as the Tillamook Estuaries Partnership, use the dike inventory to prioritize future projects.
The Tillamook Estuaries Partnership, in collaboration with Green Point Consulting and the Institute for Applied Ecology, took advantage of the inventory to aid in the creation of a strategic restoration plan. The Tillamook Estuary was divided into 92 sites and ranked according to ecological necessity, with diked locations providing the most obvious candidates for restoration. This report will prove invaluable in the rehabilitation of the Tillamook Estuary, enabling stakeholder groups to create a targeted action plan to revitalize these critical ecological habitats.
Image Credit
Jsayre64, own work. CC-BY-SA-3.0, http://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
