Watershed
The Tryon Creek watershed covers about 4,100 acres (1,700 ha) or 6.5 square miles (17 km2). Of this, about 80 percent is within the Portland city limits, and the remaining 20 percent is split among the jurisdictions of the city of Lake Oswego, and the counties of Multnomah and Clackamas. These four overlap with the jurisdiction of the State of Oregon, which owns the natural area. Watersheds that border the Tryon Creek watershed are Fanno Creek to the west and northwest, Stephens Creek to the north, the Willamette River to the east and Oswego Lake to the south. More than 70 miles (110 km) of surface streets, including parts of Interstate 5, Oregon Route 99W, Oregon Route 43, Boones Ferry Road, Taylors Ferry Road, and Terwilliger Boulevard, run through the watershed.
About 35 inches (890 mm) of precipitation, almost entirely rain, fall on the watershed each year. Summers are dry, and most of the precipitation occurs between October and May. Elevations within the drainage basin vary from Mount Sylvania, which rises to 970 feet (300 m) above sea level in the West Hills, to 10 feet (3.0 m) where the creek enters the Willamette River. Between 60 and 75 percent of the slopes in the watershed exceed a 30-percent grade and are especially steep near the headwaters. Impervious surfaces cover about 24 percent of the basin. These surfaces and the relatively impermeable silt and clay soils that underlie the area contribute to rapid runoff and low baseflows in the creek and its tributaries. The total length of surface streams in the drainage basin is about 27 miles (43 km), while another 3 miles (5 km) run through culverts or pipes. Although major flooding in 1996 caused landslides and severe damage to stream beds and banks along Tryon Creek and its tributaries, it caused no significant property damage in the watershed.
In 2000, the population of the Tryon Creek watershed within Portland was about 18,000. In 2005, Portland's Bureau of Environmental Services (BES) described land-use zoning in the watershed as follows: single-family residential, 55 percent; parks and open space, 14 percent; multi-family residential, 5 percent; commercial, 3 percent, and insufficient data, 2 percent. To reach 100 percent, BES listed but did not differentiate by zoning type the roughly 20 percent of the watershed that lies beyond the Portland city limits. Metro, the regional government for the Portland metropolitan area, says that parks and natural areas cover about 21 percent of the total watershed, while single-family housing dominates most of the rest.
Read more about this topic: Tryon Creek