page_title: Johnson Creek (Willamette River tributary)
section_title: Hydrology
hierarchical_section_title: Johnson Creek (Willamette River tributary) / Watershed / Hydrology
caption_attribution_description: Johnson Creek almost in line with SE 42nd Street where a small island is formed by the creek branching (seen rejoining here). Also a weir is visible just off the Springwater Corridor.
context_page_description: Johnson Creek is a 25-mile tributary of the Willamette River in the Portland metropolitan area of the U.S. state of Oregon. Part of the drainage basin of the Columbia River, its catchment consists of 54 square miles of mostly urban land occupied by about 180,000 people as of 2012. Passing through the cities of Gresham, Portland, and Milwaukie, the creek flows generally west from the foothills of the Cascade Range through sediments deposited by glacial floods on a substrate of basalt. Though polluted, it is free-flowing along its main stem and provides habitat for salmon and other migrating fish.
  Prior to European settlement, the watershed was heavily forested and was used by Native Americans of the Chinook band for fishing and hunting. In the 19th century, non-Native American settlers cleared much of the land for farming, and the stream is named for one of these newcomers, William Johnson, who in 1846 built a water-powered sawmill along the creek. By the early 20th century, a rail line parallel to the stream encouraged further residential and commercial development. As urban density increased in the floodplain, seasonal floods grew more damaging.
context_section_description: The watershed can be divided into two hydrologic areas with different infiltration rates. The northern area, comprising about 40 percent of the watershed, consists of the Portland Terraces, and the southern consists of the Boring Hills and the Kelso Slope. Most of the rain that falls on the northern area percolates into the ground, and most of the rain that falls on the southern area runs quickly into the creek or its tributaries. Rain that falls on the north generally has a more gradual, longer-lasting effect, sustaining the base flow of the creek during dry periods.
  About 40 percent of the tributaries that originally flowed over the surface of the watershed were piped or relocated during urban development, especially on the north side of the main stem. Generally, the remaining free-flowing tributaries begin south of the main stem and run north; the major exception is Crystal Springs Creek, which begins as a groundwater discharge and flows south. The other major tributaries are Hogan, Kelley, Butler, Sunshine, and Badger creeks. Crystal Springs and Kelley creeks contribute more flow to the main stem than the other tributaries.
  Fill at Foster Road and Southeast 111th Avenue usually prevents stormwater runoff from a 9-square-mile (23 km²) area of the watershed in the Lents and Powellhurst-Gilbert neighborhoods from flowing directly into the creek. Instead, runoff is routed to sumps, where it percolates into the ground. Normal drainage patterns have also been altered further downstream in the Sellwood, Eastmoreland, Westmoreland, and Woodstock neighborhoods, where runoff flows into the Portland sewer system instead of into the creek.
caption: Lower Johnson Creek rejoins itself after forming a small island near Southeast 42nd Avenue.
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