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Title: Constraining Dynamic Sediment‐Discharge Relationships in Cold Environments: The Sediment‐Availability‐Transport (SAT) Model
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Water Resources Research
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Thin layer sediment placement (TLP) is a method to mitigate factors resulting in loss of elevation and severe alteration of hydrology, such as sea level rise and anthropogenic modifications, and prolong the lifespan of drowning salt marshes. However, TLP success may vary due to plant stress associated with reductions in nutrient availability and hydrologic flushing or through the creation of acid sulfate soils. This study examined the influence of sediment grain size and soil amendments on plant growth, soil and porewater characteristics, and greenhouse gas exchange for three key US salt marsh plants: Spartina alterniflora, Spartina patens, and Salicornia pacifica. We found that bioavailable nitrogen concentrations (measured as extractable NH4+-N) and porewater pH and salinity were found to have an inverse relationship with grain size, while soil redox was more reducing in finer sediments. This suggests that utilizing finer sediments in TLP projects will result in a more reduced environment with higher nutrient availability, while larger grain-sized sediments will be better flushed and oxidized. We further found that grain size had a significant effect on vegetation biomass allocation and rates of gas exchange, although these effects were species-specific. We found that soil amendments (biochar and compost) did not subsidize plant growth but were associated with increases in soil respiration and methane emissions. Biochar amendments were additionally ineffective in ameliorating acid sulfate conditions. This study uncovers complex interactions between sediment type and vegetation, emphasizing limitations of soil amendments. The findings aid restoration project managers in making informed decisions regarding sediment type, target vegetation, and soil amendments for successful TLP projects. 
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  2. Abstract Purpose The equilibrium sediment exchange process is defined as instantaneous deposition of suspended sediment to the streambed countered by equal erosion of sediment from the streambed. Equilibrium exchange has rarely been included in sediment transport studies but is needed when the sediment continuum is used to investigate the earth’s critical zone. Materials and methods Numericalmodeling in the watershed uplands and streamcorridor simulates sediment yield and sediment source partitioning for the Upper South Elkhorn watershed in Kentucky, USA.We simulate equilibrium exchange when uplandderived sediment simultaneously deposits to the streambed while streambed sediments erode. Sediment fingerprinting with stable carbon isotopes allowed constraint of the process in a gently rolling watershed. Results and discussion Carbon isotopes work well to partition upland sediment versus streambed sediment because sediment deposited in the streambed accrues a unique autotrophic, i.e., algal, fingerprint. Stable nitrogen isotopes do not work well to partition the sources in this study because the nitrogen isotope fingerprint of algae falls in the middle of the nitrogen isotope fingerprint of upland sediment. The source of sediment depends on flow intensity for the gently rolling watershed. Streambed sediments dominate the fluvial load for low and moderate events, while upland sediments become increasingly important during high flows and extreme events.We used sediment fingerprinting results to calibrate the equilibrium sediment exchange rate in the watershed sediment transport model. Conclusions Our sediment fingerprinting and modeling evidence suggest equilibrium sediment exchange is a substantial process occurring in the system studied. The process does not change the sediment load or streambed sediment storage but does impact the quality of sediment residing in the streambed. Therefore, we suggest equilibrium sediment exchange should be considered when the sediment continuumis used to investigate the critical zone.We conclude the paper by outlining future research priorities for coupling sediment fingerprinting with watershed modeling. 
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