26 Jul Downstream benefits of peat wetland restoration on the Fleurieu Peninsula
As regular readers will know, Glenshera Swamp (including Stipiturus Conservation Park) is one of the largest and most intact examples of the critically endangered Fleurieu Pensinsula Swamps, and a site that NGT have been helping to restore in South Australia.
Back in July 2016, several months before we had undertaken any restoration works, we installed a water level logger in the drain at Saffrons Road. All flows from Glenshera pass this location, so it is ideal for measuring the effect of restoration on flows leaving the wetland. Our hydrological restoration works at Glenshera, which involved redirecting drains back into the wetland and blocking drains cut through the wetland, were completed in early April 2017. We anticipated that as restored areas of the wetland ‘refilled’ to a new level, there would be a brief drop in outflows, followed by a return to pre-restoration flow rates.
At the end of May 2019 we downloaded the Saffrons Road logger and converted the data to an average daily flow rate (megalitres per day). The results are a striking example of what wetland restoration can achieve, and far exceeded our expectations (see graph below).
Firstly, from the cumulative annual rainfall (orange bars), it is clear that 2016 was a wet year, with about 1200 mm of rainfall, and that 2017 and 2018 were drier, each with around 700 mm of rain. The average (for Mt Compass, where this rainfall was measured) is 786 mm.
Secondly, the flows out of the wetland, at Saffrons Rd (blue line), decline to a low level in summer, when rainfall is minimal. This ‘summer baseflow’ has clearly increased following restoration. A very brief decline in flow rate (only a few days) occurred immediately after the restoration works were completed, but in the summers of 2017/18 and 2018/19 summer baseflow was markedly higher than it had been prior to restoration. Of note, this baseflow was higher despite 2017 and 2018 being considerably drier than 2016.
Thirdly, the higher flow rate in the wetter months appears to persist for longer after the restoration occurred. In 2016, despite the wet winter and spring, flows had dropped to their minimum by mid-November. In 2017 flow rates reach their minimum in December and by the summer of 2018/19 elevated flow rates persisted through to January.
With its peaty substrate, Glenshera Swamp acts like a large sponge. The ‘sponge’ absorbs and stores the water entering it, and releases water when it becomes saturated. NGT’s hydrological restoration works have effectively increased the capacity and size of the ‘sponge’ (i.e. restoring the former area of peat wetland), meaning there is more water being stored during the wetter months, and therefore more is being gradually discharged during the drier months. The bigger wetland means more, and wetter, habitat for the flora and fauna of Glenshera itself. But it also means benefits downstream too. Increased summer baseflows out of Glenshera means important downstream refuge pools, for fish or other aquatic species, are less likely to dry out in the summer. Significantly for neighbours, this also means that shallow groundwater levels in the immediate vicinity of the swamp are also being buffered by these works, which means an extended dry period in the future is less likely to result in their bores running dry.
When extrapolated to the landscape scale, these results clearly illustrate the cumulative effects of swamp drainage. In drained areas, winter flows run off the country much faster than it did naturally, and less water is retained in peat wetlands for slow release during the drier months. As a consequence, watercourses are more prone to erosion in winter and complete desiccation in summer, conditions less suitable for the flora and fauna that rely upon these wet habitats.
In summary, wetland restoration can definitely have benefits well beyond the wetland itself!
And the good news is that what we have now demonstrated at Glenshera Swamp can readily be replicated in peat wetlands elsewhere across the Fleurieu and similar habitats throughout south-eastern Australia. If you are keen to talk to us about the potential to restore your peat wetland, please get in touch.
Ben Taylor and Tessa Roberts.