Watershed
Draining 31.7 square miles (82.1 km2), Fanno Creek receives water from Portland's West Hills, Sexton Mountain in Beaverton, and Bull Mountain near Tigard. Nearly all of the watershed is urban. About 7 square miles (18 km2), roughly 22 percent of the total, lies inside the Portland city limits. The highest elevation in the watershed is 1,060 feet (320 m) at Council Crest in the West Hills. The peak elevation on Sexton Mountain is 476 feet (145 m), while Bull Mountain rises to 715 feet (218 m). About 117 miles (188 km) of streams flow through the watershed, including Ash Creek, Summer Creek, and 12 smaller tributaries. A small part of the drainage basin lies in the city of Lake Oswego in Clackamas County, near the headwaters of Ball Creek, a Fanno Creek tributary.
The slopes at the headwaters of Fanno Creek consist mainly of Columbia River Basalt exposed in ravines but otherwise covered by up to 25 feet (8 m) of wind-deposited silt. Silts and clays are the most common watershed soils, and significant erosion is common. About 50 inches (1,300 mm) of precipitation, almost all of which is rain and about half of which arrives in November, December, and January, falls on the watershed each year. Although significant flooding occurred in 1977, the watershed has not experienced a 100-year flood since the area became urban.
Small watersheds adjacent to the Fanno Creek watershed include those of minor tributaries of the Willamette or Tualatin rivers. Tryon Creek, Balch Creek, and other small streams east of Fanno Creek flow down the eastern flank of the West Hills into the Willamette. To the northwest, Hall Creek, Cedar Mill Creek, and Bronson Creek flow into Beaverton Creek, a tributary of Rock Creek, which empties into the Tualatin River at the larger stream's RM 38.4 (RK 61.8), about 29 miles (47 km) upriver from the mouth of Fanno Creek.
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