Floodplain Trapping and Cycling Compared to Streambank Erosion of Sediment and Nutrients in an Agricultural Watershed

Abstract

Floodplains and streambanks can positively and negatively influence downstream water quality through interacting geomorphic and biogeochemical processes. Few studies have measured those processes in agricultural watersheds. We measured inputs (floodplain sedimentation and dissolved inorganic loading), cycling (floodplain soil nitrogen [N] and phosphorus [P] mineralization), and losses (bank erosion) of sediment, N, and P longitudinally in stream reaches of Smith Creek, an agricultural watershed in the Valley and Ridge physiographic province. All study reaches were net depositional (floodplain deposition > bank erosion), had high N and P sedimentation and loading rates to the floodplain, high soil concentrations of N and P, and high rates of floodplain soil N and P mineralization. High sediment, N, and P inputs to floodplains are attributed to agricultural activity in the region. Rates of P mineralization were much greater than those measured in other studies of nontidal floodplains that used the same method. Floodplain connectivity and sediment deposition decreased longitudinally, contrary to patterns in most watersheds. The net trapping function of Smith Creek floodplains indicates a benefit to water quality. Further research is needed to determine if future decreases in floodplain deposition, continued bank erosion, and the potential for nitrate leaching from nutrient‐enriched floodplain soils could pose a long‐term source of sediment and nutrients to downstream rivers.

Quantifying connectivity and its effects on sediment budgeting for an agricultural basin, Chesapeake Bay Watershed, United States

“…Gillespie et al (2018) for Smith Creek, VA estimated floodplain deposition ranging from 0.7 to 1.7 cm/year (average: 1.1 cm/year), higher than the 0.26 cm/year median value measured here. This is noted in their study as remarkably high, especially when compared to other noteworthy sediment budget studies for agricultural watersheds both within and outside of the Chesapeake Bay watershed.…”

“…Counties, PA (rates of 5.0 to >20 cm/year) (Walter & Merritts, 2008). Gillespie et al (2018) for Smith Creek, VA estimated floodplain deposition ranging from 0.7 to 1.7 cm/year (average: 1.1 cm/year), higher than the 0.26 cm/year median value measured here. This is noted in their study as remarkably high, especially when compared to approach is recommended by Crema and Cavalli (2018), however this contrasts with the approach originally used by Borselli et al (2008) and this study.…”

Sediment dynamics and implications for management: State of the science from long‐term research in the Chesapeake Bay watershed, USA

“…Stream reaches switched from being typically erosional in first through third‐order streams, neutral in fourth‐order, and depositional in fifth and sixth order streams of the Difficult Run watershed (Hopkins et al, 2018; Hupp et al, 2013). In contrast, Gillespie et al (2018) found that sediment balance was depositional among sites in third‐ or fourth‐order streams, but decreasingly so in larger streams, of an agricultural Virginia watershed. Pizzuto et al (2018) measured that floodplain deposition was more than three times greater than bank erosion along a fifth‐order stream.…”

“…Floodplain deposition Floodplains can trap sediment through both deposition of channel load during overbank flooding as well as by riparian buffering of sediments eroded from adjacent uplands. Floodplain deposition is spatially variable through the watershed depending on land use, geology, reach geomorphology, and floodplain hydrologic connectivity (Gellis et al, 2009;Gillespie et al, 2018;Hopkins et al, 2018;Hupp et al, 2013;Noe & Hupp, 2005;Pizzuto, Skalak, Pearson, & Benthem, 2016;Schenk, Hupp, Gellis, & Noe, 2013;Wolf, Noe, & Ahn, 2013). In general, greater rates of floodplain sedimentation occur where greater sediment load is transported by streams, greater hydrologic connectivity exists between stream channels and floodplains, and floodplain complexity is greater (like highly variable microtopography, coarse woody debris, and plant biomass).…”

“…The spatial and temporal factors that influence bank erosion are being identified. Studies that have measured bank erosion found it typically increases with stream drainage area (Gellis et al, 2015; Gellis et al, 2017; Gillespie, Noe, Hupp, Gellis, & Schenk, 2018) and where bank sediment is less dense and has less coverage by woody vegetation and roots (Wynn & Mostaghimi, 2006). Conversely, bank aggradation by riparian grasses leads to narrower stream channels compared to forested riparian zones (Hession, Pizzuto, Johnson, & Horwitz, 2003; Sweeney et al, 2004).…”