Saturday, October 20, 2007

St Lucia mouth closes - Friday 24 August 2007

St Lucia mouth closes

October 15, 2007;

St Lucia Mouth
 
Ricky Taylor
  
The St Lucia mouth closed on Friday 24 August after having been open for 175 days. This closure was expected as much sediment had accumulated in the mouth during the several months prior to closure. The closure was hastened by the windy conditions experienced during the preceding few weeks. Although the mouth is closed there has been overtopping of seawater on each high tide. It is most unlikely that the mouth will breach naturally in the near future.
 
At closure, the catchments areas for St Lucia were in the grip of a most extended and severe drought. As a result almost no freshwater was entering the system via the rivers. The water level in the lake was close to that of the sea. The salt concentration in the water in St Lucia, which had been a little less than that of seawater at closure, had increased to above that of seawater by the start of October when good rains fell in the whole region. Once again the rivers started flowing and hopefully this is an indication that good summer rains will follow.
 
Since the mouth opened in March, recruitment of marine life occurred – mainly of fish, crabs and prawn larvae. EKZNW survey shows this to have been mainly in the Narrows, but not to a great extent in the rest of the estuarine system. The present closure has been too early for the main spring recruitments.
 
However, the St Lucia system, although stressed by the drought that is now in its seventh year, is remarkably resilient. The management strategy of having kept the mouth closed during most of this drought period has shown that the quantity of salt in the system has not become excessively large. The best indicators of the vitality of the ecosystem are the birds. EKZNW counts in May and August showed 17 000 and 15 000 birds respectively. These are both regarded as high counts.
 
At present the management strategy is to allow the St Lucia Mouth to stay closed. The EKZNW will monitor the situation. If this summer brings good rains and the rivers flow strongly; then the freshwater inputs will exceed evaporation losses. Under these conditions the water level will rise and salinity will drop. It is only once the water level has risen to above that of the sea and we are assured that the drought has finally broken, that we will consider breaching the mouth to re-establish the link between St Lucia and the sea.
 
In the past the Umfolozi River and St Lucia shared a common mouth. During droughts this mouth would close, diverting Umfolozi water into St Lucia. Computer models have been developed that show that this would have lessened the impacts of drought to a huge degree. This situation cannot be reinstated at present because canals excavated to support sugar farming in the Umfolozi swampland allow the sediments to pass through what was formerly a swamp filter. Any Umfolozi water entering St Lucia nowadays would carry an unacceptably high sediment load with it. 
 
 R H Taylor
Ecologist: EKZNW
13 October 2007

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