They were separated as part of an irrigation plan in 1952.

This will restore the functioning of South Africa's biggest estuarine system while preserving its natural beauty.

As a result of the inflow of fresh water, and below-average rainfall between 2002 and 2010, the St Lucia estuary mouth to the sea was closed for the best part of the past decade.

This meant that St Lucia had not functioned as an estuary and fisheries, such as the inshore line fish and crustacean fisheries, were badly affected.

The park's CEO, Andrew Zaloumis, yesterday said the re-linking of the Umfolozi River to the St Lucia estuarine lake system was an important first step.

"Our ultimate aim in the restoration of Lake St Lucia is to protect iSimangaliso's world heritage site," he said.

To re-link the Umfolozi to Lake StLucia, iSimangaliso and Ezemvelo KZN Wildlife used a tracked excavator to establish a beach spillway (a passage for surplus water) between the two mouths in March.

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.

Zaloumis said park experts, who had been monitoring the state of the Umfolozi, observed the first closure in May.

"Due to the low level of the sand bar, the mouth opened and closed a few times during high tides," Zaloumis said.

"This movement across the sand bar served to deposit sand and slowly raise its height, eventually creating a more stable closed-mouth condition.

"The earlier preparatory work on the spillway meant that only five days of work were required to achieve the appropriate depth and length for joining the systems," said Zaloumis.

He said water levels in the system are a steady half a metre on average in the north and south lakes, and at least 1m in the narrows.