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Riparian Habitat

Historic

The riparian habitat of the Willamette River bottomland in this subwatershed was likely a mosaic of wetlands, grassland, and forested areas. In general, riparian areas support high species abundance and diversity because of the varied sources of food, water, and shelter found in these streamside habitats.

According to reconstructions of land survey records, the Miller Creek riparian areas were fully covered by a closed canopy of mixed conifer forest. The very upper end of the subwatershed is recorded as having burned sometime in the mid-1800’s. There is no indication of the composition of the riparian area within the burned areas.

Bank slumping and landslides would have been a common occurrence along Miller Creek. Slope failure would have occurred naturally because of the steep slopes and fine-textured soils present throughout the West Hills that become unstable when saturated. Soil failure can occur on slopes of 15% or more. Periodic wildfires may have exacerbated slope failure in certain situations.

Current

This subwatershed is composed almost entirely of Forest Park and undeveloped portions of Multnomah County outside of the city boundary. Consequently, above highway 30 the riparian areas along Miller Creek resemble historic conditions of closed-canopy mixed conifer deciduous forest. Based on visual estimates of 2002 aerial photography, the riparian zone of Miller Creek and associated tributaries appear to consist of about 99% trees and shrubs with a small percentage (< 1%) of residential/cleared areas. Common tree species include big leaf maple, vine maple, willow, western hemlock, red alder, western red cedar, and Douglas fir.

In the bottomland, the short reach of stream below Highway 30 has sparse riparian vegetation with poor habitat value and dominated by grasses and small deciduous trees. Remaining riparian wetland habitat in the bottomland also includes an area referred to as the Harborton Wetlands Site adjacent to the Willamette just south of the mouth of Miller Creek (City of Portland BES, 2000). This resource site consists of a series of riparian patches (some remnant) interspersed with industrial development. A forested wetland at the northern end is still connected to the floodplain. The shrub layer is relatively sparse. Tree canopy cover within the bottomland forest ranges from patchy cover (20 percent canopy closure) to relatively dense cover (80 percent closure). The structural diversity of the forest is relatively high. Trees vary in age from approximately 40 to 60 years. Snags and large woody debris are common, particularly along the riverbank and beach. With the exception of one riprapped section, the shoreline is natural with beaches throughout the length of the resource site.

Assessment

With the exception of a very small percentage of development, the current riparian quality reflects historic conditions. Although historic information is lacking, current channel and riparian habitat conditions above Highway 30 are expected to be very similar to conditions in Miller Creek prior to Euro-American settlement.

As residential and urban land-use and road construction becomes more common (particularly in the lower creek), riparian habitat conditions will decline and become more fragmented.
Maps & Files

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