Add sediment: freshwater marshes

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • One study evaluated the effects, on vegetation, of adding sediment to existing freshwater marshes. The study was in the USA.


  • Overall richness/diversity (1 study): One replicated, randomized, paired, controlled study in the USA reported that adding sediment to freshwater marshes typically reduced plant species richness after one growing season.


  • Overall abundance (1 study): One replicated, randomized, paired, controlled study in the USA found that adding sediment to freshwater marshes had no significant effect on total live vegetation biomass after one growing season.
  • Individual species abundance (1 study): The same study found that adding sediment to freshwater marshes had no significant effect on the biomass of most of the dominant herbaceous species after one growing season.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. 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
Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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Marsh and Swamp Conservation

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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