Plug/dam canals or trenches: freshwater marshes

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

50%
Certainty

30%
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

One study evaluated the effects, on vegetation, of plugging/damming canals or trenches in freshwater marshes. The study was in the USA.

VEGETATION COMMUNITY

Overall extent (1 study): One replicated, site comparison study of backfilled canals in freshwater marshes in the USA reported that emergent marsh vegetation coverage was greater within the channels of plugged than unplugged canals, after 6–60 months. However, coverage on former spoil areas did not significantly differ between plugged and unplugged canals.

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, site comparison study in 1983–1984 of five backfilled canals in freshwater marshes in Louisiana, USA (Neill & Turner 1987) reported that emergent marsh vegetation coverage was greater within plugged than open canals, but that coverage was similar on the adjacent former spoil areas. Statistical significance was not assessed. After 6–60 months, emergent vegetation coverage was 15% within plugged canals (vs <1% in open canals) and 35% on the former spoil areas alongside plugged canals (vs 35% alongside open canals). Methods: In 1983 and 1984, vegetation was surveyed in three freshwater canals that had been plugged with earth or seashell dams at one end, and two canals that had not been plugged. Coverage of emergent marsh vegetation was estimated from aerial photographs. All canals, originally dug by the oil and gas industry, had been backfilled with adjacent spoil between 1979 and 1984.
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.

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
Retain/create habitat linkages across service corridors Action Link	No evidence found (no assessment)	0	Synopsis Link
Backfill canals or trenches: freshwater swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Plug/dam canals or trenches: brackish/salt marshes Action Link	Likely to be beneficial	2	Synopsis Link