Floodplain phosphorus and sediment deposition in the West Nishnabotna River Corridor
Date
2024-12
Authors
Baah, Kelvin Adu
Major Professor
Advisor
Moore, Peter L
Isenhart, Thomas M
Miller, Bradley A
Committee Member
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Abstract
Approximately one-third of the phosphorus entering fluvial systems in Iowa originates from eroding streambanks rather than direct agricultural runoffs. Fine alluvial sediments in riverbanks, often enriched in legacy phosphorus, are transferred to surface waters upon bank erosion. In large rivers across the agricultural Midwest, most total phosphorus is transported as particulate phosphorus, following the fate of sediment. Some sediment and phosphorus can be returned to the floodplain by lateral bar accretion or by vertical accretion during overbank flow conditions. Deposition in these settings is known to be influenced by floodplain topography and widespread roughness elements like vegetation.
This study is aimed at characterizing the influence of floodplain vegetation, relative elevation or height above the river channel, and distance to the river channel on the magnitude and spatial distribution of sediment and phosphorus accretion in 30 km of the West Nishnabotna River corridor. Using geospatial analysis, 150 floodplain sites were randomly selected and surveyed in detail by sampling sediments, mapping vegetation, estimating relative elevation and distance, and sediment accretion.
Floodplain spatial features, including distance and elevation relative to the river channel, were found to have no significant effects on sediment and TP deposition. While vegetative traits were also found to have no significant effects on sediment and TP deposition, vegetation distribution had varying significant impacts on deposition on a broader scale. It was discovered that grassed areas had the highest sediment and TP deposition, followed by cropped and forested areas, respectively. The average mass of TP per channel length retained by the cropped and grassed areas are 0.442 and 0.413 kg per meter of channel length, respectively, while the forested areas erode 0.634 kg per meter of channel length. The average mass of TP per area retained by the cropped and grassed is 0.001 and 0.015 kg/m2, respectively, while the forested areas erode 0.008 kg/m2. Overall, the floodplain was found to have a net retention of 8327.56 m3 of sediment and 0.22 kg of TP per meter of channel length (0.008 kg/m2). Our results suggest that enhancing the conservation and restoration of floodplain grassed riparian zones and the best agricultural management practices can increase the functionality of floodplains as sediment and nutrient sinks during overbank flow conditions.
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