Researchers find Iowa’s farm ponds are filling with sediment faster than expected

Iowa researchers have found that farm ponds — designed to control flooding and curb the amount of soil and organic materials moving downstream — are filling up with sediment more quickly than expected.

Scientists with the Iowa Geological Survey focused their attention on farm ponds in southeast Iowa and found that the ponds are filling up with soil and other sediment 25 percent faster than they were designed to.

Keith Schilling, director of the Iowa Geological Survey, said the findings were unexpected, “especially considering that the pond watersheds were dominated by perennial vegetation.”

Most of the farm ponds the researchers studied were constructed with a 50-year life span. The ponds filling up faster means their life spans may be reduced.

“A shorter life span means that the sediment is filling the pond and reducing the volume of water that can be stored in them,” Schilling said. “Less water storage will reduce the potential for retaining floodwaters when they arrive.”

The researchers found that gully erosion from nearby woodlands and forest areas was a major contributor to sediment in farm ponds, in addition to erosion from cropland. Gully erosion is a more severe form of erosion that occurs when heavy rain forms deep gullies in soil.

  Buy Photo

Schilling said forest and grassland areas are “not immune” to gully erosion because those landscapes are “always eroding.”

“Even before agriculture arrived, hillsides were eroding and eroded sediment was filling low areas. However, this occurred over long periods of time. Erosion can be much quicker and more severe when row crop agriculture exposes bare soil during heavy rains, especially without much conservation protection,” Schilling said. “In our pond watersheds, the areas still bear the legacy of historical agriculture and the eroded landscape that existed before the ponds and vegetation were established. In forested areas, the dense canopy shades out a grassy understory which makes the land easier to erode.”

Although Schilling and his team focused their research on farm ponds in southeast Iowa, he said ponds across the state could be filling faster due to gully erosion.

Specifically, he said it is “very applicable” to other areas of the state where there are “steeply sloping“ catchments that drain into farm ponds.

Schilling said this would likely be less common in central Iowa where the land is flatter, but it could happen in areas like the Loess Hills region in western Iowa and south-central parts of the state.

“The big story here is that erosion happens even in non-cropland. In fact, forests in some areas can contribute a lot of sediment,” said Matthew Streeter, a soil scientist, who co-authored the study with IGS. “We never consider putting a terrace in wooded areas. But if we want to maintain the longevity of practices like this, we need to look at options to address nearby gully erosion as part of the original installation process.”

A need for more conservation work, additional research

Despite their findings, Schilling said that implementing farm ponds still is a key way to reduce the amount of sediment moving downstream from farms and forested areas.

“The practice of installing farm ponds remains an important strategy to reduce sediment export, but results from our study suggest that managers should examine the contributing catchments to see if there are signs of gully erosion and if so, take steps to mitigate or repair them before investing in the pond,” he said.

  Buy Photo

Schilling said it is important to reduce sediment where erosion is happening around the state.

“Management practices such as no-till or sediment erosion practices such as installation of terraces, water and sediment control structures or buffers would slow water flow and reduce sediment erosion closer to where it is generated,” he said.

Going forward, he said additional research is needed to better measure the sedimentation rates into farm ponds from different watersheds.

“There is likely a gradient of erosion and sedimentation that occurs under different conditions,” Schilling said. “Our study is an important step toward awareness that ponds are filling up faster than designed and even those with watersheds of grass or forest are not immune to the threat.”

Olivia Cohen covers energy and environment for The Gazette and is a corps member with Report for America, a national service program that places journalists in local newsrooms to report on under-covered issues. She is also a contributing writer for the Ag and Water Desk, an independent journalism collaborative focusing on the Mississippi River Basin.

Sign up for our curated, weekly environment & outdoors newsletter.

Comments: olivia.cohen@thegazette.com