Gully erosion filling up ponds faster than expected

AMES — Farm ponds in Iowa built primarily for flood-control purposes are designed for an expected life span, based on expected sedimentation rates.

New research, led by a team from the Iowa Geological Survey, set out to determine how actual sedimentation rates correlate with the ponds’ original predicted longevity.

Their research focused on a set of ponds in southeastern Iowa where the watershed is dominated by pastures, grasslands enrolled in the Conservation Reserve Program and forests. Despite the predominance of conservation land in the study area, the ponds were filling up faster than their original 50-year designs predicted.

A thorough assessment of the ponds' watersheds revealed that the major contributor of sediments was gully erosion in the nearby woodlands.

The study's findings were recently published in the Journal of Soil and Water Conservation by Matthew Thomas Streeter, a soil scientist, and co-authors Keith E. Schilling, Iowa Geological Survey director, Calvin F. Wolter, GIS specialist and Valerie Diaz-Gibertini, research assistant, all with the Iowa Geological Survey and University of Iowa.

Their exploratory project, funded by the Iowa Nutrient Research Center at Iowa State University, assessed three ponds of varying ages to quantify the amount of sediment stored within each pond and relate those quantities to original capacity estimates, assess the landscape source of sediment filling the ponds and estimate stocks of carbon, nitrogen and phosphorus stored within each pond to help identify the potential for farm ponds to reduce exports of nutrients downstream.

The researchers studied the flood-control ponds and their associated catchments in the Soap Creek Watershed in Wapello County, southwest of Ottumwa. One of the ponds was on Streeter’s family farm. The other two were on neighboring farms.

Built in 1993 and 2003, the ponds ranged from about 4 to 11 acres and were constructed with a 50-year life span.

The ponds provided a unique opportunity to quantify total sediment storage as the total pond volume was provided in the original engineering site survey and designs developed by the federal Natural Resources Conservation Service, according to Streeter.

Land use for all catchments had only changed slightly since the ponds were built. In 1993, it was predominantly perennial grass (48%), forest (35%) and cropland (13%). As of 2021, perennial grass cover had decreased to about 38% and forest increased to 42%, while cropland remained about the same.

An innovative method to analyze Iowa’s statewide LiDAR data sets was developed and implemented by Wolter, which estimated catchment-scale sediment erosion and deposition while identifying specific locations where sediment elevation changed. Wolter further analyzed current bathymetry data (the measurement of depth of water in the pond) and original pond design plans to estimate pond sedimentation rates.

On average, the research team found the ponds had filled up with sediment at rates 25% faster than design estimates and discovered that gullies in the forested land were the primary contributor of the fill materials.

“The big story here is that erosion happens even in non-cropland. In fact, forests in some areas can contribute a lot of sediment. 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,” said Streeter.

The researchers also found that the ponds were storing significant quantities of total phosphorus in the eroded sediments. They estimated the ponds could reduce the watershed P export by up to 20% during their life span.

Farm ponds are a popular conservation practice. Impoundments for small ponds and lakes have often been constructed with 100% government support to improve flood control and reduce delivery of eroded sediments from uplands to downstream waterways.

The ponds can also provide livestock watering, wildlife habitat and outdoor recreation. While individual ponds have small-scale impacts, their combined benefits can be significant.

“The decreased pond capacity we found compromises their intended purposes,” Streeter said. “Our results suggest that regardless of land use type, gully stabilization techniques like grassed waterways and terrace construction should occur before building ponds for conservation and flood control. Ideally, there should be requirements to maintain those practices over the intended pond life span. Otherwise, the benefits will be much less than projected.”