Nature's "Drainage Canal": The Columbia River Floodplain

Although the Columbia Slough is 18 miles long, its watershed covers a 61 square mile area from urban North Portland to Gresham. Prior to human manipulation, the slough's channels, lakes, and wetlands absorbed the waters of the Columbia during natural periodic spring freshets and floods. The teeming plant and animal life, and rich soils of the floodplain drew human settlement to the area.

This 1909 postcard shows the Columbia Slough connected to the Sandy and Columbia Rivers during high water. The spring flood stage sometimes created a passage way on the slough from the Sandy River inland to the Willamette. Indians navigated the waterways of the sloughs and lakes near the Columbia, traveling to Willamette Falls. White settlers and East Portland residents also traversed the slough's waters. Courtesy of Nancy Hoover

For centuries humans used the natural bounty of the Columbia floodplain. Over the past 150 years, they transformed the once-bountiful wetlands by logging, diking, and building levees.

Land along a river or stream that floods periodically when the banks overflow is known as a "floodplain."

Merriam Webster's Collegiate Dictionary defines floodplain as:

• Level land that may be submerged by floodwaters
• A plain built up by stream deposition

Floodplains are beneficial to humans because they provide:

• Highly fertile topsoil deposited by rivers
• Convenient irrigation
• Proximity to water supplies and transportation routes
• Recreational opportunities, scenic beauty, and wildlife habitat

Some negative impacts of floodplains to humans include:

• High risk of damage by flood to lands in urban areas and croplands
• Industrial and commercial development of high-risk lands
• Possible inundation of urban areas due to rising sea levels over the next 50 years
• Disproportionate impact on people living near industrial areas subject to flooding and other urban ailments because of low-property values and historic social policies

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