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The first time in recorded history that the Uilkraals Estuary became closed for a long period of
time was in January 2009. It re-opened six months later for a short period but closed again for
an extended period in December 2009. Water storage in the Kraaibosch Dam, approximately
10 km upstream from the estuary, has significantly altered the natural freshwater in-flows to
the estuary while agricultural activities in the upper catchment have introduced an increased
sediment load into the estuary, ultimately resulting in reduced flow over time and an increased
likelihood of mouth closure of the estuary in low flow periods.
The lower reaches of the estuary used to consist of several braided channels that expanded to
a single 400 m wide channel at high tide (Harrison et al. 1995b). Water in the area below the
bridge is now restricted to two smaller shallow channels, the larger of which ends at the beach
in front of the huts at the caravan park (Figure 11). Before mouth closure occurred, tidal
influence reached beyond the bridge, with the majority of the sandflats becoming inundated at
high tide (Harrison et al. 1995b). Tidal interchange was recorded up to 3 km upstream in a
1981 survey (Heydorn & Bickerton 1982). Currently, a very shallow braided channel runs
across the sandflats upstream from the bridge, probably similar to former low tide conditions.
The majority of the sandflats are now permanently exposed (Figure 12).
Figure 11. The closed estuary mouth facing upstream (left) and the area below the bridge facing
downstream (right, picture taken from the bridge), February 2010.
The middle reaches of the estuary consist of a wide meandering channel across a large
floodplain, surrounded by saltmarsh vegetation. Before the road bridge was built, the estuary
was a marine-dominated tidal lagoon. The large volumes of tidal exchange would have rapidly
reduced any effects of floods, when they did occur (Gaigher 1984). Factors affecting
circulation also affect salinity by altering the volumes of salty water entering the estuary as
well as the ratio of dilution of fresh and salt water (Clark 1977). The obstruction caused by the
road bridge would therefore have changed the circulation and hence salinity regime of the
estuary immensely. This would have been most critical during periods of freshwater flooding,
by prolonging the extension time of freshwater over tidal sandbanks. This effect would have
been most intensive in the extensive flood area on the landside of the bridge (Gaigher 1984).
Uilkraals Estuary Situation Assessment
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