Actively manage water level: freshwater swamps
Overall effectiveness category Likely to be beneficial
Number of studies: 2
Background information and definitions
This action involves active, repeated management of the amount of water in wetlands and when it is present, to mimic the natural hydrology of swamps. This may prevent excessively high or low water levels (e.g. during storm surges or droughts), or maintain wet/dry cycles that are a natural feature of many swamps (e.g. Conner & Buford 1998).
This action will usually involve some kind of water control structure: a valve, gate, sluice or pump. Water level management might aim to: mirror historical water level fluctuations or stability; manage salinity levels; increase sediment inputs; stimulate growth of desirable plant species; and/or create new wetland plant communities. Studies of “moist soil management”, “structural marsh management” and “environmental flows” along river courses all fall within the scope of this action. Caution: When managing water levels in a focal site, the effect on water levels in neighbouring sites should be considered.
Although water levels may be managed to restore or enhance habitats for waterfowl, information on the value of vegetation for waterfowl (e.g. seed production; productivity measured as CO2 exchange rates) is not summarized in this synopsis. Also, this synopsis does not include information on riparian areas that are not clearly marshes or swamps (e.g. riparian forests that require only a brief flood pulse for germination; Taylor et al. 2006).
Related actions: Raise water level to restore degraded swamps or restore/create swamps from other land uses; Lower water level to restore degraded swamps or restore/create swamps from other land uses; Facilitate tidal exchange to restore degraded swamps or restore/create swamps from other land uses; Manage water level to control problematic plants; Actively manage water level to complement planting.
Conner W.H. & Buford M.A. (1998) Southern deepwater swamps. Pages 261–287 in: M.G. Messina & W.H. Conner (eds.) Southern Forested Wetlands: Ecology and Management. Lewis Publishers/CRC Press, Boca Raton.
Junk W.J., Piedade M.T.F., Schöngart J., Cohn-Haft M., Adeney J.M. Wittman F. (2011) A classification of major naturally-occurring Amazonian lowland wetlands. Wetlands, 31, 623–640.
Taylor J.P., Smith L.M. & Haukos D.A. (2006) Evaluation of woody plant restoration in the Middle Rio Grande: ten years after. Wetlands, 26, 1151–1160.
Supporting evidence from individual studies
A before-and-after study in 2003–2004 of a freshwater wetland with marsh and swamp vegetation in Oregon, USA (Jenkins et al. 2008) found that following a managed flood/drawdown, plant diversity increased and there were changes in cover of individual plant taxa. Plant diversity was higher in the autumn after the flood/drawdown than in the autumn before (data reported as a diversity index). Of 21 plant taxa for which cover data were reported, 12 became more abundant, including knotweeds Polygonum spp. (before: 21%; after: 35%) and Pacific willow Salix lucida (before: 11%; after: 15%). The cover of seven taxa declined, including invasive reed canarygrass Phalaris arundinacea (before: 44%; after: 41%). The largest canarygrass declines occurred under regenerating tree canopies and in areas more deeply flooded during spring 2004 (see original paper for data). Methods: In 2004, a water control structure was used to restore a more natural water regime to a floodplain wetland: high winter and spring water levels (flooding some surveyed areas) followed by summer drawdown (exposure to natural tides). Over the previous 20 years, the water level had been artificially stabilized and reed canarygrass had invaded. Vegetation was surveyed around the edge of the wetland, in the autumn before (2003) and after (2004) the managed flood/drawdown. Plant species were recorded at 10 cm intervals along 27 transects (approximately 25,000 total points sampled), spanning marshy areas (open, herbaceous) and swampy areas (with a tree canopy). The study does not generally separate results from the two habitat types.Study and other actions tested
A site comparison study in 2006–2007 of two forested floodplains in South Carolina and Georgia, USA (Lee et al. 2016) found that an artificial flood pulse increased the number of tree seedlings in one of two forest types, but had no significant effect in the other. No data were reported for these results. The Savannah River floodplain was artificially flooded in spring 2006 by releasing water from an upstream reservoir, but was not flooded in spring 2007. In cypress-tupelo swamp forest, the number of tree seedlings/plot did not significantly differ between years. In bottomland hardwood forest (higher up on the floodplain), the number of tree seedlings/plot was greater in summer 2006 than summer 2007. The nearby Altamaha River floodplain experienced natural floods in spring 2006 and 2007. Here, the overall number of tree seedlings/plot did not significantly differ between years for both forest types. Methods: Tree seedlings were counted in July–September 2006 and 2007, in around 50 permanent 30-m2 plots/river/year.Study and other actions tested