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TRACKING CHANGES IN WETLANDS WITH URBANIZATION: SIXTEEN YEARS OF EXPERIENCE IN PORTLAND, OREGON, USA
Citation:
Kentula, M E., S. E. Gwin, AND S. M. Pierson. TRACKING CHANGES IN WETLANDS WITH URBANIZATION: SIXTEEN YEARS OF EXPERIENCE IN PORTLAND, OREGON, USA. WETLANDS. The Society of Wetland Scientists, McLean, VA, 24(4):734-743, (2004).
Description:
Long-term studies of the wetland resource in urbanizing areas are essential to understanding the effects of urbanization on wetlands and the effectiveness of management actions. Using data from the National Wetland Inventory (NWI) in combination with field surveys, we tracked changes over 16 years in small ( 2 ha), palustrine emergent/open water wetlands (PEM/POW) in the rapidly urbanizing area of Portland, Oregon, USA. These were the wetland types most often involved in permitted losses of freshwater wetlands and required as mitigation under Section 404 of the US Clean Water Act. Wetlands identified on NWI maps and that had not been converted to other land uses at the time of our 1992 survey were surveyed in 1998. Data on location, wetland type, surrounding land use and condition, and on the cause of loss were collected on 164 of the 171 wetlands in the target population. As indicated by GIS analysis of the NWI data, the field survey confirmed that 40% of the wetlands had been altered prior to 1982. Despite development pressure throughout the 1990s, loss of small PEM/POW wetlands slowed between 1987 and 1998 with only 6% of the sites being destroyed as compared to 40% lost between 1982 and 1992. Of 11 sites that were gone due to drought in 1992, eight had recovered with the return of typical rainfall; three had been destroyed. Most of the wetlands existing in 1998 were in HGM classes atypical to the region due to human manipulation. Hydrologic modifications were observed on 60% of the sites, but on-site disturbances like mowing, dumping, and trails had decreased since the 1992 survey. Over the time period studied, land use in the vicinity of the wetlands shifted from undeveloped and agricultural to urban and residential uses. Reflecting the common occurrence of on- and off-site stressors and modifications, we rated the condition of only 11% of the sites as good with 46% fair and 43% poor. Our results demonstrate the utility of field surveys and GIS analysis in tracking the status of the wetland resource over time, especially in rapidly urbanizing areas. The next challenge is to use such data to develop strategies to manage urban wetlands in ways that maintain and ultimately improve the condition of the resource.