Positive results in St Lucia estuary restoration

THE largest wetland rehabilitation project in South Africa is well underway with the St Lucia estuary restoration.

While estuary water levels remain shallow in northern regions, they were sustained during last year’s winter months by local rainfall measuring about 250mm, and river input.

Scientists are keenly monitoring the status of the lake following restoration activities and the latest information is available.

‘Restoration of the estuary has continued since the completion of the major intervention, which was to remove dredge spoil between the Mfolozi River and estuary, with the decommissioning of the estuary car park and removal of the toilet block,’ said Nicolette Forbes of Marine and Estuarine Research (MER), iSimangaliso’s contracted marine ecologist.

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‘This was done in recognition of, and as preparation for, the dynamic nature of the estuary mouth as it settles into its original configuration.’

The major driver of the system’s winter water levels was input from the Mfolozi River following its newly widened linkage behind the beach barrier, feeding water into the estuary via the lower Narrows.

These inflows drove the consistently higher water levels in the lower parts of the estuary during winter and through to early summer.

Conversely, a hot and windy summer has yielded less than 300mm to date and resulted in wind-related changes in water levels and evaporation.

Rising levels in the Narrows have occurred since September and are expected to eventually penetrate into the lake.

Rising water levels in the northern areas would have been augmented by any inflows from the rivers entering the northern parts of the estuary, including the Mkhuze, Mzinene, Hluhluwe and Nyalazi catchments.

Water levels

The various water measuring stations throughout the estuary record the levels in the different regions.

Water in the Narrows is above mean sea level and these higher levels have supplemented the southern lake levels.

Despite the estuary mouth being closed since 2014, Forbes said a number of over-washing events have occurred.

‘What is needed now is a strong freshwater pulse from the Mfolozi, of a volume large enough to overtop the sand barrier or flow with enough velocity to break through and begin the process of establishing a new mouth.’

An accumulation of sediment in the estuary’s lower reaches through to Honeymoon Bend are as a result of freshwater input from the Mfolozi and was anticipated during the early stages of restoration.

‘In the long term, once the Mfolozi has had a chance to raise water levels in the estuary and begin working through the sand barrier, outflows and tidal flushing will result in a net loss of this fine material from the lower reaches of the estuary,’ said Forbes.

Low salinity conditions have led to the aquatic plant Stuckenia pectinata colonising areas of the southern lake and Fanie’s Island.

This plant thrives in stable water conditions with salinities below 20.

Forbes said the algae and other invertebrate fauna supported by these plants create important feeding and shelter habitats for a wide variety of juvenile fish.

‘As we move through late summer, we remain hopeful that good rains will be received to speed up the restoration process, and we watch with interest as we monitor changes as the estuary returns to its natural configuration.’

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