Uilkraals Estuary - a sudden shift

By Sue Matthews, Overstrand Estuary Management Coordinator  

Just to the east of Gansbaai is the Uilkraals River estuary, a popular holiday destination in summer, when the Uilenskraalmond Resort on its western bank is packed to capacity. A management plan for the estuary is currently being developed by Cape Town-based Anchor Environmental Consultants under contract to the Cape Estuaries Management Programme, a partnership programme involving national, provincial and local government. The new Integrated Coastal Management Act identifies the need for such estuary management plans (EMPs) to ensure the integrated and coordinated management of estuaries. 

The EMP development process is initiated with a scoping phase involving consultation with stakeholders to identify the main issues affecting the estuary. This culminates in the preparation of a Situation Assessment, a report covering aspects such as the biophysical and socio-economic environment, the exploitation of living resources, water quality and quantity issues, legal requirements and institutional structures. 

On 26 August, the Anchor project team presented the draft Situation Assessment to a large group of stakeholders that included government officials, local landowners, environmentalists, anglers and other interested parties. The issue of most concern to everybody present is that the estuary mouth, which in the past remained open all year round apart from a few brief closure events, closed for an extended period in January 2009, opened briefly that winter, and has since remained closed. With winter coming to an end, it is now unlikely to open naturally without a major unseasonal storm. 

This has had a significant impact on the estuarine ecosystem. The Uilkraals estuary has extensive areas of sandflats and saltmarshes, which used to undergo daily inundation and exposure as the tide ebbed and flowed. Since the mouth closure, these areas were first exposed for months on end during summer, when river flow was confined to a few shallow channels, and then submerged as the estuary filled up with winter rains. The highly productive saltmarshes will die back under these conditions, while the sandprawns and other benthic invertebrates on the sandflats that are adapted to an intertidal existence will have either died, moved off or burrowed deeper into the sediment. 

This has had a severe knock-on effect on the estuary’s birdlife. The numbers of migratory waders, particularly Curlew Sandpipers, which forage in intertidal areas have dropped dramatically, while fish-eating birds, notably Common and Sandwich Terns, have also declined. Estuaries provide important habitat for fish, and an ichthyological survey conducted at the Uilkraals in 2006 recorded 11 species, 9 of which can be considered partially or completely dependent on the estuary for their survival. The most abundant species was Knysna sand goby, followed by harder, although sandshark contributed most to the total biomass in the system. White steenbras, which depends on estuaries as nursery areas for at least the first year of its life, was recorded here in previous surveys in 1955 and 1981. 

The big question, of course, is why has the mouth closed, given that rainfall has remained in the normal range? The knee-jerk response has been to blame the Kraaibosch Dam, built 10 km upstream in 1999 to supply Gansbaai and the surrounding agricultural areas with water. The operating rules of the dam stipulate that all summer flow is released and only a portion of winter flow is held back, but it’s highly likely that there is some dampening of flood peaks that would help scour out the estuary. However, as the farmer whose property surrounds the dam pointed out at the meeting, the mouth closure in January 2009 was just two months after a major storm caused considerable flood damage in the area on 12 November 2008. It’s quite possible, then, that the closure has instead been caused by massive amounts of sediment being washed downstream. 

Other negative impacts on the Uilkraals system include alien infestation of the catchment area, sewage pollution from the District Municipality’s oxidation ponds on the western bank, and flow diversion and obstruction caused by a road bridge embankment constructed 800 m from the mouth in 1973. The latter is believed responsible for the disappearance of bloodworm from the estuary, probably due to reduced water exchange and the effects of freshwater flooding. 

These and other issues will be addressed in the Estuary Management Plan, which may recommend artificial breaching of the mouth as a mitigation measure. An Estuary Management Forum, made up of representatives of all relevant government authorities at the national, provincial and local level as well as stakeholder groups, will then be set up to oversee the implementation of the EMP.   

Sue is contracted to the Overstrand Municipality in a position funded by The Table Mountain Fund, an associated trust of WWF-SA.