Eroding streambanks contribute a third or more of the phosphorus export from the state, according to estimates from groundbreaking research that could influence future priorities for nutrient reduction efforts in Iowa and beyond.
Nutrients in rivers and streams, consisting primarily of nitrogen (N) and phosphorus (P), contribute to hypoxic conditions in the Gulf of Mexico, and impaired water quality in lakes and other bodies of water in Iowa and around the world. The Iowa Nutrient Reduction Strategy, established in 2013, has a goal of lowering the total quantity of N and P leaving the state by 45%.
Streambanks are recognized as a potentially large contributor to phosphorus loads, but are usually unaccounted for in nutrient reduction strategies. When Iowa's nutrient reduction strategy was developed in 2013, its science assessment stated that while streambank erosion was known to be a potentially large source of phosphorus-carrying stream sediment, it was not addressed due to a lack of data. As a result, phosphorus from streambank erosion has been largely ignored in efforts to reduce nutrient loss from Iowa's watersheds.
Streambank erosion that carries sediment-bound phosphorus downstream is a natural process, but modern land development and farming practices like drainage and stream channelization can accelerate the rate of erosion. These rates tend to be highly variable and episodic, depending on weather and the severity of rainfall events.
The new study offers the first estimate of the total amount of phosphorus coming from eroding streambanks in Iowa. The project was led by Keith Schilling, state geologist and director of the Iowa Geological Survey at the University of Iowa, and Tom Isenhart, professor of natural resource ecology and management at Iowa State University, working with co-investigators Calvin Wolter, Geographic Information Systems analyst, Iowa Department of Natural Resources; Matthew Streeter, soil scientist, Iowa Geological Survey; and John Kovar, research soil scientist, USDA Agricultural Research Service. Funding for the project came primarily from the Iowa Nutrient Research Center at ISU. A peer- reviewed article about the research is in press at the Journal of Soil and Water Conservation and scheduled to appear online in November.
The research team used a complex process that combined detailed aerial mapping, computer modeling and data from on-the-ground monitoring. They started by creating a rough equation that incorporated the essential variables: the total lengths of eroding banks, bank heights, identified rates of recession (or erosion), and the bulk density and phosphorus content of the streambank soil.
To fill in a framework for analysis, they developed a data collection strategy informed by LiDAR mapping and extensive field monitoring in 16 watersheds around the state. They also drew on relevant information from team members' prior research that included years of data on Iowa streambank recession. When they calculated the rate of erosion over an 18-year period from 2000 to 2017, the long-term annual average was approximately 7,680 metric tons of total phosphorus eroding from streambanks — about one-third of the phosphorus load Iowa is sending downstream.
That estimate is conservative, according to the researchers, as their assessment only included larger, third- through sixth-order streams that could be identified by satellite mapping. For now, due to the mapping limitations, they left out smaller, first- and second-order headwater streams. These smaller streams make up most of the linear length of Iowa's stream networks but tend to have less severe bank erosion.
Combining all criteria, they estimated that 41% of streambanks in Iowa are severely eroding. As expected, more streambank erosion was found in southwest and southern Iowa than other regions of the state where the topography is flatter, bank heights are lower and soils tend to be more coarse-textured.
"Our study is believed to be among the first to address the issue of streambank phosphorus at a scale that is relevant to a statewide nutrient reduction strategy," Schilling says. "Despite limitations in the analysis, we can now say with more certainty and specificity that eroding streambanks do, indeed, contribute a large portion of the phosphorus moving to the Mississippi River, and to our lakes within the state."
What does this mean for prevention or mitigation?
According to Iowa Nutrient Research Center Director Matt Helmers, one of the authors of the Iowa Nutrient Reduction Strategy, "This study is critically important to our long-term strategies for nonpoint efforts. The focus of implementation efforts so far has largely been on land-based conservation practices. This work emphasizes the need to give greater attention to integrated practices that address in-stream sources, and reduce the volume and power of water leaving the landscape, which can ultimately stabilize streambanks and channels. We can't expect quick results, but it could make a huge difference over the long term."