FLOODS AND FLOODPLAINS
Floods raise many concerns for communities living along major rivers such as the Willamette River, but these natural processes also are important for the formation of floodplain lands, deposition of rich floodplain soils, and creative of river habitats.
The Willamette River has experiences numerous floods over the last 150 years. The floods of 1861 and 1890 covered an area of more than 320,000 acres. This flood extent is the best scientific source for delineating the functional floodplain of the Willamette River.
During the recent foods of 1964 and 1996, the Willamette River fully occupied its historical floodplain in the lower, narrow river and occupied most of the historical floodplain in the middle section of the river. The flood boundaries of the recent floods were substantially less than the extent of the historical floodplain in the upper section of the Willamette River. This is caused by 1) the lower flows in recent floods, 2) dampening of peak flows by flood control reservoirs, and 3) channel alteration and revetments. As a result, floodplain function was diminished and human property was protected.
Excerpted with permission from Gregory, S., L. Ashkenas, S. Jett, R. Wildman (2002.) Flood Inundations/FEMA Floodplains. In D. Hulse, S. Gregory, and J. Baker (Eds.). Willamette River Basin planning atlas, 2nd. Edition, (p.28.). Corvallis: Oregon State University Press
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