Project Report
Project Lead/Researchers:
Peter Moore, adjunct assistant professor, Natural Resource Ecology and Management
River corridors deliver the environmental byproducts of agriculture downstream, but can also serve as buffers to that delivery. Low-lying floodplains adjacent to rivers can be a sink for fine sediment and sediment-bound nutrients like phosphorus (P) if floodwaters inundate them and if conditions on the floodplain promote deposition and/or trapping. Riparian and floodplain vegetation is expected to play an important role in sediment and P trapping, but details of that role remain unknown. A key aspect of that uncertainty concerns the physical traits of the various plant species commonly found in floodplains. Which species are found in different parts of the floodplain and how often are they inundated with water? How broad and dense is their foliage when under water? Are their stems rigid, or do they bend easily so that the plants “lie down” in the current? These are the questions the research team sought to answer as a component of a larger of sediment and P transport in the Nishnabotna River basin.
With support from the Leopold Center and USDA McIntire-Stennis capacity funds, graduate student Kelvin Adu Baah (M.S. Environmental Science, 2024) and two field assistants visited 150 sites randomly distributed across the floodplain along a 10-mile reach of the West Nishnabotna River in southwest Iowa in the summer of 2023. At each site, they collected sediment samples and surveyed the vegetation communities in several different ways. They then identified the species and abundance (% ground cover) of all plants found within a 1 square-meter quadrat. To characterize the foliage height and density, they used a “cover pole” where visual obstruction of a colored pole was assessed from pre-determined height and distance. Stem rigidity was evaluated using the very low-tech Eastgate board-drop test, where a plywood panel of known weight and dimensions starting with one edge resting on the ground was tipped onto the vegetation. The final resting height of the board edge above the ground is closely related to the ability of the plants to remain upright under a strong current.
The researchers divided sites into broad vegetation communities, including crops (soybean or corn), grass, and forest. More than 50 plant species were identified across all sites, but each community type was dominated by one (crops) or just a few species. While ground cover, foliage obstruction, and rigidity individually did not correlate well with sediment and P trapping on the floodplain, a composite variable that we could call a “flow resistance index” did. In general, grassed sites had larger values of this index and also trapped more sediment and P per-site than forested sites. Because the annual crops are often absent during spring floods, the team concluded that grassed sites – though they covered the smallest area (less than 6% of the floodplain) – were the most effective at trapping fine sediment and P from floodwaters. Broader planting of tall grasses in hydrologically-connected floodplains can therefore potentially contribute to reductions in downstream export of sediment and P.