May Rains Cause Severe Erosion in Iowa

June 10, 2004

Heavy rains throughout May caused severe soil erosion on more than a million of Iowa’s 26 million acres of cropland, according to reports from the USDA’s Natural Resources Conservation Service (NRCS). Damage was reported across the state, with western Iowa cropland and portions of central Iowa fields most affected by the storms.

Hard rains on the long, steep slopes of western Iowa resulted in more than half the state’s acreage of severe erosion. Nearly 800,000 acres in western Iowa showed signs of severe soil erosion. The area received two to nine inches of rain, spanning four days in late May.  

Acting NRCS State Conservationist Dennis Pate said it is agency policy to ask all field offices after major storm events to estimate the number of acres where severe erosion has occurred.

“We consider severe erosion to be an area where 20-tons per acre or more of soil has detached and transported to a point of deposition,” said Pate. “It’s equivalent to an eighth of an inch of soil loss over the surface of an acre.” The average soil loss per acre per year on Iowa cropland is almost five tons.

A stretch of counties from Plymouth County in northwest Iowa to Audubon County to Madison County in southern Iowa reported some of the most severe erosion.

Despite the heavy rains and severe soil erosion, conservationists said there were fields where conservation practices held up well.

Audubon County District Conservationist David Brand said fields were pounded with back to back rains throughout the whole month of May. Brand said it was easy to see where the best conservation practices were implemented.

“No-till held up better than the conventionally tilled fields,” said Brand. “Some of the older terraces and waterways that were silted in ran over the top and ran outside of the grassed waterways.”

Though western Iowa was generally affected the most by May rains, eastern Iowa sustained damage as well. In Clayton County in northeast Iowa, District Conservationist David Gibney reported parts of the county were hit with up to ten inches of rain.

“Waterways were damaged if the seeding was not adequately established or if they were too narrow or shallow,” he said. “No-till and seeded practices held up quite well.”

Pate said the May storms tested grassed waterways in particular.

“We’ve had many reports of water running alongside a grassed waterway rather than down the middle as it should,” he said. “This year’s storms re-emphasize the point that we have to construct grassed waterways to their proper width and depth.”

The consensus among district conservationists is that fields with complete conservation systems, not just one practice, were the ones with the least damage.

“Complete conservation systems are the best answer for protection against severe soil erosion,” said Pate. “A combination of no-till with a variety of conservation practices, such as terraces, grassed waterways and contour buffer strips, perform the best in reducing erosion and allowing for water infiltration.”

Hard rains like the ones Iowa sustained last month can have long-term impacts. NRCS State Soil Scientist Mike Sucik said Iowa typically had 16 to 18 inches of topsoil just 100 years ago.

“Now the soil thickness is less than 10 inches in many areas of Iowa,” said Sucik. “A lot of that loss is directly attributed to soil erosion from water.”

Along with agricultural land across the state, developing urban areas showed signs of erosion problems. Conservationists noted bare, exposed soil in sloping areas of construction tended to create dramatic rills on slopes. Even on flatter grades, such as yards, sediment runoff into streets was widespread.

“Typically, erosion rates on construction sites can be five to ten times higher than on agricultural lands,” said NRCS Urban Conservationist Wayne Petersen. “Sediment from a construction site that enters a storm sewer has a much more direct route to receiving waters, so the water quality impact can be significant.”

-30-

NRCS Contacts:
Dennis Pate
Acting State Conservationist
Phone: 515-284-6655

Jason Johnson
Public Affairs Specialist
Phone: 515-323-2701
Email: Jason.r.johnson@ia.usda.gov

< Back to 2004 News Releases