Johnson Creek (Willamette River) - Floods

Floods

By removing the original vegetation, rural and urban development of the Johnson Creek watershed induced more rapid storm runoff and expensive floods. As a flood-control measure in the 1930s, the Works Progress Administration of the federal government cleared and lined with rock about 90 percent of the creek between its mouth and Southeast 158th Avenue, about 15 miles (24 km) from the mouth, creating an artificial channel 6 feet (2 m) to 10 feet (3 m) deep and 25 feet (8 m) to 50 feet (15 m) wide. Although substantially altering the stream and its aquatic life, this channel failed to prevent overflows, the largest of which damaged 1,200 structures in 1964.

Precipitation patterns in the Johnson Creek watershed have contributed to frequent high flows and floods along the creek, typically between November and February. Based on records from 1961 to 1990, Portland's average annual precipitation, as measured at Portland International Airport along the Columbia River, is about 36 inches (910 mm). About 21 inches (530 mm) falls from November through February and only about 4 inches (100 mm) from June through September. The airport, at about 30 feet (9 m) above sea level, is at essentially the same elevation as the mouth of Johnson Creek. Annual precipitation at higher elevations in the creek's upper watershed is much higher, more than 70 inches (1,800 mm).

The floods primarily affect four areas in Portland: Tideman-Johnson Park at Southeast 45th Avenue, the area west of Southeast 82nd Avenue; Lents, and lower Powell Butte. The U.S. National Weather Service defines Johnson Creek's flood stage, measured at USGS station 14211500 at Sycamore in Portland, as 11 feet (3.4 m), equivalent to a flow rate (discharge rate) of about 1,200 cubic feet (34 m3) per second. The USGS peak streamflow data collected by the stream gauge at that station show that the creek reached or exceeded 1,200 cubic feet (34 m3) per second 37 times between 1941 and 2006; that is 37 floods in 65 years. Twenty of those floods exceeded 1,500 cubic feet (42 m3) per second. At least seven of the floods caused major property damage. A more recent overflow occurred in December 2007, when the creek crested at 1.5 feet (0.46 m) above flood stage. Between 1978 and 1997, flood insurance claims totaling an estimated $2 million were paid for damage along the creek.

The biggest flood measured at Sycamore, 10.2 miles (16.4 km) from the mouth of the creek, occurred in 1996. Exceeding the official flood stage of 11 feet (3.4 m) by more than 4 feet (1.2 m), the creek reached 15.30 feet (4.66 m) on November 19 of that year. The second biggest flood occurred on December 22, 1964, when the creek reached 14.68 feet (4.47 m) at Sycamore.

As of 2008, the USGS was monitoring Johnson Creek at Regner Road in Gresham, 16.3 miles (26.2 km) from the mouth, and at Milwaukie, 0.7 miles (1.1 km) from the mouth, as well as at Sycamore. It also had stream-monitor stations on Kelley Creek and Crystal Springs Creek. The Sycamore station was the oldest, having begun operation in 1941.

Proposals by agencies such as Metro, the regional government of the Portland metropolitan area, and the U.S. Army Corps of Engineers in the 1970s and 1980s to control Johnson Creek flooding were abandoned after groups of residents objected to every proposal. In 1990, the City of Portland formed the Johnson Creek Corridor Committee from multiple agencies and citizen groups with varied interests related to the watershed. Led by the city's Bureau of Environmental Services, the combined groups in 2001 produced the Johnson Creek Restoration Plan to reduce flooding, improve water quality, and improve fish and wildlife habitat. Goals varied from section to section along the creek and included controlling storm water runoff, reducing erosion, replacing or mitigating impervious surfaces, and protecting riparian zones. Through 2007, at least 75 site-specific restoration projects had been carried out in the Johnson Creek Watershed, ranging from the $1.2-million Brookside Project, a constructed wetland, to small riparian repair projects. Most involved voluntary citizen participation in all phases, including the long-term management and care of the sites. In late 2012, the city completed the East Lents Floodplain Project, which restored 70 acres (28 ha) of the natural floodplain to a low-lying area along the creek.