Backfill canals or trenches: freshwater marshes

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Three studies evaluated the effects, on vegetation, of backfilling canals or trenches in freshwater marshes. All three studies were in the USA. There was overlap in the canals used in two of the studies.

VEGETATION COMMUNITY

  • Overall extent (3 studies): Three replicated studies in freshwater marshes in the USA reported coverage of emergent marsh vegetation between 6 months and 25 years after backfilling. All three studies reported that coverage was greater on former spoil areas alongside canals than within the partly filled canal channels.
  • Relative abundance (1 study): One replicated, paired, site comparison study in a freshwater marsh in the USA reported that in levelled former spoil areas alongside backfilled canals, the relative abundance of some key plant species differed from natural marshland. Vegetation was surveyed three years after backfilling.

VEGETATION ABUNDANCE

 

VEGETATION STRUCTURE

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 study in 1983–1984 of five backfilled canals in freshwater marshes in Louisiana, USA (Neill & Turner 1987) reported that they all developed some coverage of marsh vegetation, but mainly alongside rather than within the channels. After 6–60 months, emergent marsh vegetation coverage was 27% in former spoil areas alongside the channels, on average (range 20–62% for individual canals) but only 6% within the channels, on average (range <1–26% for individual canals). The study suggests that some of the variation between canals was related to the quality of the backfilling/skill of the dredge operator. Methods: The area of marsh vegetation alongside and within five backfilled freshwater canals was estimated from aerial photographs taken in 1983 and 1984. The canals, originally dug by the oil and gas industry, had been backfilled with adjacent spoil between 1979 and 1984. This reduced their water depth to 0.4–1.4 m. Three of the canals had also been plugged at one end with earth or shell dams. Four canals in this study were also studied in (2).

    Study and other actions tested
  2. A replicated study in 2000–2004 of five backfilled canals in freshwater marshes in Louisiana, USA (Baustian & Turner 2006) reported that they all developed some coverage of marsh vegetation, but mainly alongside rather than within the channels. Between 20 and 25 years after backfilling, emergent marsh vegetation coverage was 80% in former spoil areas alongside the channels, on average (range 5–95% for individual canals) but only 5% within the channels, on average (range 0–55% for individual canals). The study suggests that marsh vegetation coverage on spoil banks was related to how much of the spoil bank was actually levelled to marsh elevations. Methods: The area of marsh vegetation alongside and within five freshwater canals was estimated from aerial photographs and field surveys in 2000 and 2004. The canals, originally dug by the oil and gas industry, had been backfilled with adjacent spoil between 1979 and 1984. Between 5 and 100% of the spoil heaps alongside each canal were levelled, and the canals were made shallower (but not filled completely). Some canals were plugged at one end with earth or shell dams. Four canals in this study were also studied in (1).

    Study and other actions tested
  3. A replicated, paired, site comparison study in 2005 of two backfilled canals in a freshwater marsh in Louisiana, USA (Baustian et al. 2009) reported that they both developed some marsh vegetation within three years, but with a different relative abundance of key plant species to natural marshes. Statistical significance was not assessed. Three years after backfilling, marsh vegetation coverage was 65% on former spoil areas but only 20–25% within each canal. The relative abundance of plant species differed between former spoil areas and adjacent natural marshes. In particular, alligatorweed Alternanthera philoxeroides was more dominant on former spoil areas (23–37% of vegetation) than in natural marsh (6–9% of vegetation). The opposite was true for spikesedge Eleocharis sp. (former spoil areas: 030%; natural marsh: 2373%). Methods: In early 2002, two shipping canals were dammed and adjacent spoil was returned to the channels. One canal received additional sediment from a nearby lake. The canals were not completely filled and adjacent spoil areas were not entirely levelled. In 2005, aerial photographs were taken to estimate vegetation coverage. Vegetation was also surveyed in ten 1-m2 quadrats/canal: five on former spoil areas (including marsh and non-marsh vegetation) and five in adjacent undisturbed marsh.

    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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