Plug/dam canals or trenches: brackish/salt marshes

Overall effectiveness category Likely to be beneficial
Number of studies: 2
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How is the evidence assessed?

Effectiveness

60%
Certainty

40%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

Canals or trenches contribute to marsh and swamp degradation by allowing water of the ‘wrong’ salinity to enter (typically salty water entering freshwater or brackish sites, when canals or trenches are dug in coastal areas; LCWCRTF 2002), allowing increased water flow or tidal exchange (thus increasing rates of erosion), and/or by allowing water levels to fluctuate. Simply constructing a plug or dam at the mouth of a canal or trench, using materials such as wood or oyster shells, might reduce or solve these problems (NFWF 1995; LCWCRTF 2002).

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.

Related actions: Backfill canals or trenches; Fill/block ditches not associated with service corridors; other actions to Divert/block/stop saltwater inputs or Divert/block/stop freshwater inputs.

^{LCWCRTF (2002) Point Au Fer Canal Plugs (TE-22). Louisiana Coastal Wetlands Conservation and Restoration Task Force. Available at https://www.lacoast.gov/reports/gpfs/TE-22.pdf. Accessed 15 January 2020.}

^{NFWF (1995) FY1995 Fisheries and Wildlife Assessment. National Fisheries and Wildlife Foundation, USA.}

^{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

Two studies evaluated the effects, on vegetation, of plugging/damming canals or trenches in brackish/salt marshes. Both studies were in the USA. There was overlap in the canals used in the studies. Both studies included some freshwater areas in some analyses, but all results are based predominantly on canals in brackish or saline marshes.

VEGETATION COMMUNITY

VEGETATION ABUNDANCE

Overall abundance (2 studies): Two replicated, site comparison studies studied emergent vegetation of backfilled canals in the USA. One study reported that plugged canals had greater coverage of emergent marsh vegetation than unplugged canals after 6–60 months. One study found that emergent vegetation coverage on former spoil heaps did not significantly differ alongside plugged and unplugged canals after 6–11 years. The first study also reported that plugged canals were more likely to contain floating/submerged vegetation than unplugged canals.

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, site comparison study in 1983–1984 of 27 backfilled canals of varying salinity in Louisiana, USA (Neill & Turner 1987) reported that plugged canals had greater coverage of emergent marsh vegetation than open canals after 6–60 months, and were more likely to contain submerged vegetation. Statistical significance was not assessed. Considering 17 canals in brackish and saline marshes, emergent vegetation coverage was 7% within plugged canals (vs <1% in open canals) and 53% on the former spoil areas alongside plugged canals (vs 36% alongside open canals). Considering 27 canals of varying salinity (but mostly brackish or saline), 92% of plugged canals contained floating or submerged aquatic vegetation (vs 43% of open canals). Methods: In 1983 and 1984, vegetation was surveyed in up to 13 canals plugged with earth or seashell dams at one end, and 7–14 canals that were not (or no longer) plugged. Coverage of emergent and floating vegetation was estimated from aerial photographs. Submerged vegetation was identified in ground surveys. All canals, originally dug by the oil and gas industry, had been backfilled with adjacent spoil between 1979 and 1984. This study selected canals from the same master set of 33 used 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, site comparison study in 1990 of 23 backfilled canals predominantly in brackish and saline marshes in Louisiana, USA (Turner et al. 1994) found that plugging had no significant effect on marsh vegetation coverage alongside the canals. After 6–11 years, coverage of emergent marsh vegetation on former spoil areas alongside canals did not significantly differ between plugged canals and open canals. However, there was a trend towards lower coverage alongside plugged than open canals. Methods: In 1990, aerial photographs were taken of 23 canals. All canals had been backfilled with adjacent spoil between 1979 and 1984. The mouths of some canals (number not reported) had also been plugged with earth or seashell dams at one end, to maintain water levels and reduce saltwater inputs. The area of the former spoil heaps covered by marsh vegetation was determined from the photographs. This study selected canals from the same master set of 33 used in (1). The study does not separate results for freshwater, brackish and saline marshes, but most canals (approximately 80%) were in brackish or saline marshes.
Study and other actions tested
Referenced paper
Turner R.E., Lee J. & Neill C. (1994) Backfilling canals to restore wetlands: empirical results in coastal Louisiana. Wetlands Ecology and Management, 3.

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?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Backfill canals or trenches: brackish/salt marshes Action Link	Likely to be beneficial	4	Synopsis Link
Backfill canals or trenches: freshwater swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Plug/dam canals or trenches: brackish/saline swamps Action Link	No evidence found (no assessment)	0	Synopsis Link