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The link between the uMfolozi and Lake St Lucia system has been restored after 60 years.
In 2011 iSimangaliso publicised its strategy to let the uMfolozi River and Lake St Lucia rejoin in a bid to restore the functioning of South Africa’s largest estuarine system. Since 1952 the uMfolozi River has been deliberately kept separate from the St Lucia system, which has reduced freshwater inflow to the system and interfered with natural mouth dynamics. These factors, combined with below average rainfall conditions during the 2002 – 2010 period, have resulted in the St Lucia estuary mouth remaining closed to the sea for the best part of the last decade. This has meant that St Lucia has not functioned as an estuary and fisheries, such as the inshore line fish and crustacean fisheries, have been heavily impacted. The relinking of the uMfolozi river back to the St Lucia estuarine lake system is an important first step towards the restoration of estuarine function. Since the beginning of summer the uMfolozi has migrated naturally northwards from Maphelane towards St Lucia. As winter approached and water flows in the uMfolozi dropped this migration slowed and the uMfolozi reached to within 300 m of the St Lucia mouth. This placed the system in a good position for iSimangaliso to facilitate the linking of the two systems (see picture below).
The uMfolozi estuary migrated steadily northwards during the summer and slowed to within 300 m of the St Lucia mouth.
To achieve this, iSimangaliso, together with Ezemvelo KZN Wildlife, used a tracked excavator to establish a beach spillway between the two mouths during early March 2012. The spillway was prepared as far west on the beach as possible along a route previously followed by the uMfolozi River where there is a natural slope from the uMfolozi to the St Lucia system (see pic below). This entailed the removal of old dredger spoil and marine sediments. The process of spillway construction was carefully monitored by Park ecologists and iSimangaliso GEF project specialists, including an assessment of the optimal time for rejoining. Due to the beach profile, it was necessary for the uMfolozi mouth to be closed prior to the final linking of the system. Also, a closed uMfolozi river mouth would lead to water flowing into the St Lucia system through the dry winter period. The final stage of constructing the spillway was left until the uMfolozi mouth closed.
The position of the spillway between the two estuary mouths.
The state of the uMfolozi mouth was closely monitored by Park staff and the first closure was observed on the 25th May 2012. Due to the low level of the sandbar the mouth opened and closed a few times during high tide events. This movement across the sandbar served to deposit sand and slowly raise its height eventually creating a more stable closed mouth condition.
Maintenance and final preparation of the spillway began again in earnest on the 2nd July 2012 (see pic below). The earlier preparatory work on the spillway meant that only five days work was required to achieve the appropriate depth and length to join the systems.
The removal of the sandplug between the uMfolozi river and St Lucia system occurred on the 6th July 2012. This formally relinked the two systems since their separation in 1952. By early Monday morning water levels within the uMfolozi/uMsunduse system had dropped by 25 cm providing a real indication of the effectiveness of the spillway in carrying water into the St Lucia system.
The conditions in the combined system will continue to be closely monitored during this important time. Surveys of the system conducted by Oceanographic Research Institute (ORI) in May 2012 show that the rainfall through late summer and autumn has lowered and maintained salinities around or below that of sea water (35) throughout the system (dark blue line in graph below). Data from the November 2011 survey by CRUZ from the University of Zululand and ORI is shown for comparison (light blue line). The salinity level in North Lake currently varies from slightly above sea water in the north to about 19 in the south (Dead Tree Bay). South lake from north to south has fairly constant lower values of 5. The salinity in the Narrows has increased since November as a result of the influence of marine water entering through the backchannels during each high tide while the uMfolozi river has been open to the sea. The current salinity profile from north to south still represents a reversed salinity gradient not normally found in South African sub-tropical estuaries.
Water levels in the system have remained steady with levels of 0.5 m on average in North and South Lakes and at least 1 m in the Narrows.
The spillway will provide much needed water for the St Lucia system; ultimately lifting lake levels and contributing towards the longer term plan to restore the estuarine functioning of Lake St Lucia which comprises about 60% of South Africa’s estuarine resource. “Our ultimate aim in the restoration of Lake St Lucia is to protect iSimangaliso’s World Heritage Site without setting up a management regime that includes continuous manipulation” says iSimangaliso CEO Andrew Zaloumis.
This process is part of the iSimangaliso Wetland Park Authority’s development of long-term solutions to improve the hydrological functioning of Lake St Lucia through the iSimangaliso/Global Environmental Facility (GEF) Project.
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