The St Lucia Estuary Process of Rehabilitation

The St Lucia Estuary Process of Rehabilitation

Brief Overview of events

The beach spillway, created to expedite the reconnection of the uMfolozi River with the Lake St Lucia estuary, was opened on 6 July 2012.

Between 6 July and 7 September 2012, the water flow through the spillway, as well as the water levels and salinity within the Lake St Lucia estuarine system have been monitored closely by park staff. The monitoring has indicated that the spillway was extremely successful in transferring a large volume of water from the uMfolozi catchment to Lake St Lucia while the mouth was closed.

The volume of water transferred through the spillway raised the water level in the Lake St Lucia Narrows by 33 cm. Once levels were raised, the water began to flow northwards from the 20 km long Narrows into Makakatana Bay and the southern portion of the lake.

Spring Rains

Early spring rains began with an intense and rapid-forming cut-off low pressure system centred over the eastern part of the country on 5 September 2012.

This weather system resulted in significant amounts of rain falling throughout the park during 4 – 7 September 2012.

The uMfolozi River water level rose rapidly and brought increased volumes of water through the spillway for a period of 24 hours before breaching directly into the sea. The spillway was substantially widened and deep water flowed bank-to-bank into Lake St Lucia Estuary.

By uMfolozi standards this event was a relatively small flood (a so-called one-in-five year flood). The steady rain falling over the catchment lifted the uMfolozi water levels over a 24 – 36 hour period. Rather than bursting out to sea along the most direct route, the rising water levels overtopped the beach sand barrier at a low point and the river opened to the sea in the early hours of 8 September 2012.

Breaching of the uMfolozi

Once the uMfolozi River breached, water flowed southwards from the Estuary through the beach spillway and out through the open mouth. This occurred until the water level in the Estuary dropped to levels below the base of the beach spillway.

In the final account, the water level in the Narrows has risen by a total of 25cm, with a corresponding increase in the rest of the Lake, and levels are the highest they have been since November 2011. In this configuration it is likely that the beach spillway will continue to intermittently connect the uMfolozi, Lake St Lucia and the sea.

The extent to which this will occur is dependent on sea condition and wave action, the water levels in the Lake and the uMfolozi river and tidal influence.

Image Source: iSimangaliso Wetlands Authority; Pelicans at Charters Creek

Since these first spring rains broke the winter dry period further rains have been received over the weekend of 14–16 September 2012. These have resulted in good rainfall being received across iSimangaliso.

Groundwater Recharge

These two rainfall events have brought much needed water to fill wetlands and pans, and to recharge groundwater levels. Rivers like the uMkhuze, uMphathe and Nyalazi that flow directly into the Lake are also flowing strongly.

Combined with the effect of direct rainfall on the Lake, salinities throughout the system have decreased substantially with the highest salinity values in the north of the system at 53 parts per thousand (Listers Point) and 2 parts per thousand in the Narrows.

The current configuration of the uMfolozi and Lake St Lucia mouths, connected by the beach spillway, marks the beginning of a period of marine connectivity. Sampling carried out on either end of the spillway by researchers from the University of Zululand in August 2012, confirm this.

Marine Life Flourishing

Juvenile fish that entered the uMfolozi River through its open mouth last year were found to have moved through the beach spillway into the Lake St Lucia system. These fish species are estuarine dependent species that need estuaries to complete their life cycle.

Prawns and estuarine dependent fish species like grunter, stumpnose and cob spawn at sea and come into the estuary when they are small juveniles (approximately half a centimetre in size). They leave the estuary as sub-adults. Estuarine residents like goby are spawned and remain in the estuary for their whole life cycle.

Park staff will continue to monitor the fishing effort in the mouth area and members of the public are requested to respect conservation measures implemented to assist the fish and invertebrate stocks to recover.

Bird Life

Biannual monitoring that takes place as part of the national Coordinated Water Bird Counts (CWAC) project shows that healthy numbers of many species of birds currently use the Lake St Lucia system, with 35 000 water birds counted during the last monitoring trip undertaken by Park staff.

For example, flamingos are currently found in the northern parts of the lake in large numbers indicating the presence of shallow brackish water that supports the type of food they eat. White pelicans are also present in large numbers and have bred successfully with more than 700 large chicks found in this sheltered breeding colony.

There is also a large number of bird species using the intertidal areas at the open mouth for feeding and resting.

Migratory waders such as curlew sandpipers, whimbrels and ringed plovers have recently arrived from the Palaearctic for the summer. They are feeding on invertebrates living in the sand and mudflats, such as worms and small crustaceans.

Pelicans and herons are feeding in the shallow water on fish and prawns. Large flocks of terns, predominantly swift terns but also including little terns, Caspian terns and the rarer vagrant Sooty terns are also found in large numbers.

The sooty tern has drawn a lot of interest from bird-watching enthusiasts and has been reported on the South African Rare Bird report over the past few months.

Article Source: iSimangaliso Wetlands Authority

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