Johnson Creek is a tributary, about 25 miles (40 km) long, of the Willamette River in the Portland metropolitan area. Part of the drainage basin of the Columbia River, its watershed consists of 54 square miles (140 km2) of mostly urban land occupied by about 175,000 people as of 2006. 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 covered with Oregon ash, alder, and western redcedar forests and scattered black cottonwood groves in riparian areas; Douglas-fir and Oregon white oak grew in the uplands. Native Americans of the Chinook band fished and hunted in this area, and they used fire to maintain prairie openings for native plant foods such as camas. In the 19th century, the watershed was taken over by white settlers who altered the landscape. The stream is named for one of these newcomers, William Johnson, who in 1846 built a water-powered sawmill in what later became the Lents neighborhood of Portland. Data from the United States Geological Survey (USGS) show that the creek reached or exceeded flood stage 37 times between 1941 and 2006. The creek's watershed includes the subwatersheds of Badger Creek, Sunshine Creek, Kelley Creek, Mitchell Creek, Veterans Creek, Crystal Springs Creek, and smaller streams. Parks along the creek and its tributaries include natural areas, a wildlife refuge, a rhododendron garden, a botanical garden, and a 21-mile (34 km) bicycle and pedestrian rail trail that follows the creek for much of its length.