Raise water level to restore/create freshwater swamps from other land uses

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

Study locations

Key messages

  • Two studies evaluated the effects, on vegetation, of raising the water level to restore/create freshwater swamps from other land uses or habitat types. Both studies monitored the effects of one river dechannelization project in the USA.

VEGETATION COMMUNITY

  • Overall extent (1 study): One before-and-after study of a floodplain in the USA reported that after dechannelizing a river to raise the water level, the area of shrubby and forested wetlands increased – reaching greater coverage than before intervention, but also than before degradation.
  • Community types (1 study): The same study broke down overall swamp coverage into specific community types. For example, most of the shrubby wetlands that developed after raising the water level were dominated by a non-native species – which was not present historically.

VEGETATION ABUNDANCE                                                                            

  • Overall abundance (1 study): One before-and-after, site comparison study of historical shrubby wetlands on a floodplain in the USA reported that dechannelizing a river to raise the water level reduced overall vegetation cover in the following nine years.
  • Characteristic plant abundance (1 study): The same study reported that after dechannelizing a river to raise the water level, only one of two sites became dominated by wetland-characteristic shrubs. The other site remained dominated by wetland-characteristic herb species.
  • Individual species abundance (1 study): The same study reported that dechannelizing a river to raise the water level slightly increased cover of buttonbush Cephalanthus occidentalis in one of two sites (no data for other site).

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 before-and-after study in 1954–2008 in Florida, USA (Spencer & Bousquin 2014) reported that after dechannelizing a river to rewet the floodplain, the area of shrubby and forested wetlands increased. After roughly 3–8 years of rewetting, shrub-dominated wetlands covered 17–18% of the floodplain (vs 3–4% in a degraded state before rewetting, and 1% in the natural state before degradation). Most of the shrubby wetland area after rewetting was dominated by invasive Peruvian water primrose Ludwigia peruviana, which was not present before degradation. Mixed shrubby/herbaceous wetlands covered 7–18% of the floodplain after rewetting (vs 3–15% before rewetting and 52% before degradation). Coverage of forested wetlands was also greater after rewetting than before rewetting or degradation (data reported as maps). In total, wetland vegetation (shrubby, forested, herbaceous and submerged) covered 65–83% of the floodplain after rewetting (vs 22–37% before rewetting and 84% before degradation). Methods: Between 1999 and 2001, Section C of the Kissimmee River was dechannelized. This restored its natural meandering course, raised the water table on the adjacent floodplain and allowed for seasonal floods. Floodplain vegetation was mapped from aerial photographs taken before degradation (1954), during degradation (1974, 1996) and after restoration (2003, 2008). This study used the same rewetted floodplain section as (2).

    Study and other actions tested
  2. A before-and-after, site comparison study in 1998–2010 in Florida, USA (Toth 2017) reported that dechannelizing the river to rewet the floodplain had mixed effects on vegetation across two sites that were historically swamps. Statistical significance was not assessed. In the year before rewetting began, one restoration site (higher elevation) was dominated by shrubs: mostly upland (46–49% cover) but some wetland-characteristic (9% cover). The other restoration site (lower elevation) was dominated by wetland-characteristic herbs (71% cover). Total cover was 68–93%. Over roughly nine years after rewetting was complete, only the higher site had substantial cover of wetland-characteristic shrubs (3–29%). Canopy cover of habitat-characteristic buttonbush Cephalanthus occidentalis was <1–6% (vs before: 1%). The other site was dominated by wetland-characteristic herbs and floating/submerged plants. In both sites, vegetation cover after rewetting was highly variable across seasons and years (e.g. wetland-characteristic herbs: 1–82%; floating/submerged plants: 0–54%; overall: 1–92%). Over the entire study period, vegetation cover was relatively stable in another part of the floodplain that remained drained: a mixture of wetland and upland herbs (32–62% cover) and shrubs (8–34% cover). Methods: Between October 1999 and February 2001, Section C of the Kissimmee River floodplain was rewetted by dechannelizing the river. Eighteen 100-m2 plots were established in parts of the floodplain that were historically buttonbush swamps (more recently drained and grazed/overgrown). There were 12 plots in the dechannelized section and six in an upstream section that remained channelized. Plant species and their cover were surveyed in spring and summer before intervention (1998–1999) and for roughly nine years after (until 2010). This study used the same rewetted floodplain section as (1).

    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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