Backfill canals or trenches: freshwater marshes

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 3
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How is the evidence assessed?

Effectiveness

50%
Certainty

30%
Harms

2%

Calculating overall effectiveness category

Background information and definitions

Backfilling involves returning dredged or excavated material to a canal (e.g. dug for boat traffic) or trench (e.g. dug for pipelines). Sometimes additional soil or sediment is brought in if there is not enough excavated material left. In theory, backfilling restores more natural wetland conditions: the water depth in the canal and the height of adjacent spoil heaps are both reduced. The whole area then has more natural water levels and may support desirable marsh or swamp vegetation. Backfilling a canal will usually prevent boats from using it too. The success of this action may depend heavily on the skill of the operator, e.g. their ability to create the desired water/soil elevations and avoid overcompacting the fill material. Turner et al. (1994) estimated that backfilling canals in Louisiana cost US$1.20/m³ (US$1.98 corrected to 2017).

For this action, as throughout the synopsis, we have only summarized results that are solely or predominantly related to the specified habitat. For example, the results in Turner et al. (1994) combine data from approximately 80% brackish or salt marshes and 20% freshwater marshes – so they have not been summarized as evidence for freshwater marshes.

Evidence summarized for this action relates to effects on vegetation within or immediately adjacent to canals or trenches, dug as or associated with service corridors.

^{Turner R.E., Lee J.M. & Neill C. (1994) Backfilling canals to restore wetlands: empirical results in coastal Louisiana. Wetlands Ecology and Management, 3, 63–78.}

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

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
Referenced paper
Neill C. & Turner R.E. (1987) Backfilling canals to mitigate wetland dredging in Louisiana coastal marshes. Environmental Management, 11, 823-836.
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
Referenced paper
Baustian J.J. & Turner R.E. (2006) Restoration success of backfilling canals in coastal Louisiana marshes. Restoration Ecology, 14, 636-644.
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: 0–30%; natural marsh: 23–73%). 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
Referenced paper
Baustian J.J., Turner R.E., Walters N.F. & Muth D.P. (2009) Restoration of dredged canals in wetlands: a comparison of methods. Wetlands Ecology and Management, 17, 445-453.

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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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Plug/dam canals or trenches: brackish/saline swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Backfill canals or trenches: freshwater marshes Action Link	Unknown effectiveness (limited evidence)	3	Synopsis Link
Backfill canals or trenches: freshwater swamps Action Link	No evidence found (no assessment)	0	Synopsis Link