Add sediment: freshwater marshes
Overall effectiveness category Awaiting assessment
Number of studies: 1
Background information and definitions
Adding small amounts of sediment to marshes is a possible action to counter multiple threats. These include: sea level rise; subsidence (e.g. following oil and gas extraction); reduced sediment inputs following the construction of levees, flood control structures or jetties; and erosion from storms or boat traffic or following excessive grazing (Reed & Wilson 2004). Adding sediment can physically raise the ground surface and provide nutrients to vegetation. In turn, vegetation can physically protect and stabilize wetlands, and encourage further sediment deposition. Sediment or sediment slurry could be added directly to a focal site, or placed nearby then transported to the focal site by natural process (Foster 2013).
Factors that might influence the effects of this action include the amount of sediment added, and whether any vegetation is present before sediment addition.
Related actions: Deposit soil/sediment to form physical habitat structure; Transplant or replace wetland soil in order to introduce marsh vegetation.
Foster N.M., Hudson M.D., Bray S. & Nicholls R.J. (2013) Intertidal mudflat and saltmarsh conservation and sustainable use in the UK: a review. Journal of Environmental Management, 126, 96–104.
Reed D.J. & Wilson L. (2004) Coast 2050: a new approach to restoration of Louisiana coastal wetlands. Physical Geography, 25, 4–21.
Supporting evidence from individual studies
A replicated, randomized, paired, controlled study in 2004 in two floating freshwater marshes in Louisiana, USA (Carpenter et al. 2007) found that adding sediment reduced plant species richness, but had no significant effect on vegetation biomass. After one growing season, plots amended with sediment had lower plant species richness than unamended plots in five of six cases (for which amended: 8–12 species/0.4 m2; unamended: 12–13 species/0.4 m2; statistical significance not assessed). Sediment addition had no significant effect on total, live, above-ground vegetation biomass (amended: 270–660 g/m2; unamended: 320–530 g/m2). Sediment addition typically had no significant effect on the overall biomass of dominant plant species, such as slender spikerush Eleocharis baldwinii and dotted smartweed Polygonum punctatum (see original paper for data). However, in one of two marshes, biomass of frogfruit Phyla lanceolata was greater in amended plots (4–12 g/m2) than unamended plots (<0.1 g/m2). Methods: In spring 2004, thirty-two 1-m2 plots were established across two floating marshes. Sediment inputs to the marshes had been reduced by an upstream dam. In each marsh, twelve random plots were amended with sediment collected from a nearby river channel (2 kg/m2, 7 kg/m2 or 17 kg/m2). The remaining plots received no sediment. All plots were also fenced to exclude nutria Myocastor coypus. In autumn 2004, vegetation was cut from 0.1 m2 of each plot then separated by species, dried and weighed.Study and other actions tested