Second phase of hydrological restoration works underway at Hesperilla Conservation Park

NGT’s long-term involvement with Square Waterhole Swamp, within Hesperilla Conservation Park has continued recently, thanks to funding from the National Parks and Wildlife Service (NPWS). Hesperilla, named for the extremely rare golden haired sedge-skipper (Hesperilla chrysotricha), is an important example of the critically endangered Fleurieu Swamps ecological community. The site is located south of Mount Compass on SA’s Fleurieu Peninsula.

NGT’s 2017 hydrological restoration plan for Hesperilla has guided our work at this reserve. To date, this has involved remediation of a major drain and several associated minor drains at the southern end of Hesperilla in 2023. These works have been complemented by a prescribed burn by NPWS in autumn 2023, ongoing weed management by the Friends of Stipiturus and Hesperilla Conservation Parks and revegetation supported by both organisations and the Hills and Fleurieu Landscape Board. Adjoining landholder Andrew Peresano has generously fenced an area of his property adjoining the Conservation Park and supported restoration works, effectively increasing the size of the swamp. In exciting news for conservation, a breeding pair of endangered Mount Lofty Ranges southern emu-wrens were recently observed within the area of the 2023 prescribed burn, close to our 2023 hydrological restoration works.

With NPWS planning a prescribed burn at Hesperilla for autumn 2026 (it may have already occurred by the time this article is published), the opportunity arose for the next phase of hydrological restoration works to take place.

On 26th March, I was joined on site by our regular earthworks contractor Craig Haywood to undertake remediation of the main drain that passes diagonally through the centre of Hesperilla and is the most significant remaining hydrological legacy of historic attempts to drain this swamp. Unsure of how accessible the area would be due to the permanently waterlogged status of this groundwater-fed swamp, we used Craig’s smallest, rubber-tracked excavator. We also deliberately undertook this work prior to the prescribed burn so that the dense ‘mat’ of vegetation itself would help prevent the machine from sinking into the peat.

Thankfully the machine was able to traverse all the way to the most low-lying and wettest end of the drain, where it intersects with the southern boundary of the reserve, and from there we worked back up-slope and upstream. A distinct spoil mound created when the drain was first excavated prior to 1949 was still evident and clearly affecting surface water movement. The drain itself was over 1m deep in places and carrying water. Due to several decades of oxidation and compaction of this spoil bank, there was insufficient material to completely backfill the drain, so we used the spoil that was available to place a series of 11 complete blocks in the drain along its length.

A few days later Mark was able to send up the drone for a birds-eye view of the works. Encouragingly, the works footprint, the area where the excavator had removed the old spoil mound, had by then become a series of shallow (c.5 cm) pools of water, indicating that the blocking of the drain had lifted the level of surface water in the vicinity. This was the intention, to lift the water surface, keep the surface peat hydrated and retain water in the swamp for longer, i.e. to reinstate the hydrological regime that would have existing prior to the drain being dug. Over the longer term this should help maintain the ecological values of Hesperilla.

Putting the drone up also provided an opportunity to have a look at how the autumn 2023 hydrological restoration works and prescribed burn area are recovering. I think you’ll agree from the images below that things are looking very positive. Note the expansion of the swamp into Andrew Peresano’s property in the bottom right of the images.

Another question NGT and NPWS have been pondering relates to the small dam in the north-west corner of Hesperilla that was excavated into the water table some time prior to 1972. It has been unclear if this dam, although unnatural, has been protecting any ecological values that are worthy of being retained. In particular, we were interested to know if there was a resident native fish community. NGT’s aquatic ecology field crew, Cory and Ida, set nets on the evening of 15th April and checked them the following morning. The catch consisted of three adult redfin (Perca fluviatilis) an introduced species that is an aggressive predator of native fish. It would be near impossible to eliminate redfin from the dam and retain it for use as a surrogate site for threatened native fish. Thus, from a native fish perspective, the dam has no current value.

Following the prescribed burn, the plan is to continue with the drain remediation works. Burns in Fleurieu Swamps nearly always reveal hidden drains, the legacy of historic attempts to drain and convert these swamps to agriculture, with ongoing negative implications. In the case of Hesperilla, we have looked in detail at historic aerial imagery and know that a number of drains will be revealed and will be more easily remediated with the dense swamp vegetation temporarily removed by the burn. So, stay tuned for further updates.

Hydrological restoration investigations and works at Hesperilla Conservation Park were funded by the National Parks and Wildlife Service, South Australia.