Willamette River - Flooding

Flooding

Due to the volume and seasonality of precipitation in its valley, the Willamette River has often flooded. In 1861, rainstorms and warm temperatures combined with a well-above-average snowpack in the Cascades created the largest Willamette River flood in recorded history. An observer of the flood wrote, "The whole Willamette valley was a sheet of water". From Eugene to Portland, thousands of acres of rich riverside farmland were washed away and many towns in the valley were damaged or destroyed. The Great Flood, as it is sometimes called, was massively destructive to human development because most of that development was located in the river's floodplain. Floodplain locations offer easy access to river transportation and the best soils for farming. The 1861 flood peaked at 635,000 cubic feet per second (18,000 m3/s)—which was more than the flow of the Mississippi River—and inundated some 353,000 acres (1,430 km2) of land.

In the summer of 1866, the Willamette was measured at 21 feet (6.4 m) above the "low water mark," and there were more flooding worries. This raised the level of the Columbia River and nearly flooded The Dalles, a city on the Columbia.

The floods returned in early 1890, when the river first rose very quickly and then fell very quickly. Portland's main street was completely submerged. Communication over the Cascades was cut off, and many rail lines were forced to shut down. In 1894, another major flood occurred on the Willamette, and although it too caused huge damage, it was not quite as large as that of 1861.

In the 1940s the Willamette once again flooded its valley. It destroyed five bridges in Lane County in December 1942, caused seven deaths in Portland and forced people in Eugene to evacuate in January 1943, caused minor flooding in Corvallis in November 1946, and nearly flooded parts of Salem in December 1948.

Although the Willamette was, by mid-century, heavily engineered and controlled by a complex system of dams, channels, and barriers, it experienced severe floods through the end of the century. Storms caused a major flood that swelled the Willamette and other rivers in the Pacific Northwest from December 1964 through January 1965, submerging nearly 153,000 acres (620 km2) of land. Before dawn on December 21, 1964, the Willamette reached 29.4 feet (9.0 m), which was higher than the Portland seawall. By this time, about 15 people had died as a result of the flooding and about 8,000 Oregonians had been forced to evacuate their homes in search of other shelter.

On December 24, President Lyndon B. Johnson ordered federal aid for the flooded areas. Meanwhile, the Willamette continued to rise. In the next couple of days, the river receded, but on December 27, it was at 29.8 feet (9.1 m), which was still nearly 12 feet (3.7 m) above the flood stage. The Willamette continued to pose flooding threats through January 1965, and more stormy weather occurred along the Pacific Coast.

In February 1996, very heavy warm rains driven by a subtropical jet stream fell on a high, water-heavy snowpack in the Willamette watershed. These conditions, similar to those that caused the 1861 flood, caused some of the costliest flooding in the river's recorded history. An Associated Press journalist wrote, "The river crested at one town after another—at Corvallis 3½ feet above flood stage, Oregon City 18 feet above, Portland 10.5 feet above—much like a meal moving through a boa constrictor." The flood was serious enough to interrupt the progress of Oregon's growing economy, but the inundated acreage was smaller than in 1964—only about 117,000 acres (470 km2).

About 450 concrete flood-protection walls in Portland that had been constructed during the February flood and each weighed about 5,500 pounds (2,500 kg) were removed in April. In October, they were replaced by a larger steel wall that cost the city about $300,000. The new wall had 0.25-inch (6.4 mm) removable steel plates designed to better prevent future flooding.