The Willamette plan area includes the portions of Portland that drain directly into the river. It is highly urbanized, and most of the floodplain has been filled and developed. Many of the natural surface streams have been diverted into the sewer system, and impervious surfaces cover a large portion of the plan area.
Topography distinguishes the east and west sides of the Willamette River. The west side is characterized by the Tualatin Mountains (commonly referred to as the West Hills) rising from a narrow terrace along the Willamette River. The east side is relatively flat with little elevation change except for a few volcanic buttes such asMt.Taborand Rocky Butte.
Consequently the east side has been almost completely developed, and the small streams that once crossed the area have been diverted into the sewer system (Johnson Creek, at the City's southern boundary, is the only remaining free-flowing stream). The steeper slopes in the West Hills developed more slowly, and most ofPortland's remaining open stream channel can be found on the west side. With a few exceptions such as the Oaks Bottom complex, most of the natural riparian areas and wetlands on both sides of the river were filled over the past 150 years.
The west side also has significantly more parks and open space, primarily because of Forest Park. The combination of existing stream channel and open space provide greater opportunity for watershed rehabilitation on theWillamette's west side.
The majority of the area is served by combined sewers, although some neighborhoods have been completely or partially separated. In areas served by separate storm sewers , runoff is discharged to the Willamette River either untreated or after flowing through a pollution reduction facility, typically a retention system. Both the east and west sides of the Willamette River have a relatively high proportion of impervious surface relative to total area.
Seasonal high and low water flows have been attenuated by dam operations in the upper Willamette basin, and those plant and animal communities that depended on water level fluctuations have suffered as a result. The total contribution to river flow fromPortland's watershed planning area is insignificant compared to instream volume.
The marked reduction in peak flows has reduced the size and frequency of flood events critical for maintaining habitats. Dredging, channel straightening, and bank treatments have changed the channel resulting in a loss of complexity and shallow water habitat. Extensive development in the floodplain and alterations in channel banks have destroyed or degraded floodplain and off-channel habitats. Degraded water quality is the result of more than 150 years of development throughout the Willamette River basin. Bacteria, oxygen, copper, lead, and temperature are the primary water quality parameters of concern. Although it will not be possible to restore the urban area to predevelopment conditions, it is important to understand and describe the reference conditions that represent a healthy watershed.