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The Swartvlei estuary Part 1

The Swartvlei mouth will not be breached this month (November 2017), unless in an event of further rain and water levels reach 2m for the Swartvlei estuary.

Opening and closing estuary mouths-Swartvlei and Touw River

Estuaries have many natural states (open and closed, high and low levels of inflow of both fresh and marine waters. During Spring high tides, the upper estuary is filled to a high water level which causes strong outflow during spring low water.’ Howards Williams & Allanson, 1979.

Both the Swartvlei and Touw River systems have estuaries and estuarine lakes. Estuaries are globally understood to be ‘enclosed coastal bodies of brackish water (resulting from a mixture of freshwater and seawater) with one or more rivers or streams flowing into them, with a connection to the open sea.

Why open and close the mouth of the estuary? (also called breaching)

The presence and height of a sandbar determines open/closed phases of the mouth of the estuary. During open phases on high tides Water moves from the sea to the estuary ensuring a combination of both fresh and sea water. This is important for fish species dependent on the opening of the estuary mouth in order to facilitate recruitment of larvae and post-larvae into the system from the sea.

Closed phases are natural and can be highly productive for a variety of estuarine biota including plants, zooplankton, macrobenthos and fish.

This allows a whole range of juvenile forms of estuarine-dependent and estuarine-associated marine species to be recruited inside their sheltered and productive reaches, where they complete their growth to maturity.’

Whitfield and Bate (2007) indicate ‘annual migrations of postlarvae into temporarily opened and closed estuaries TOCEs ‘indicate that these life stages act as the dispersal

phase for this species, allowing it to spread into areas where it is otherwise unable to breed successfully. Mud prawn larvae require a marine phase of development and are often absent from intermittently open estuaries (IOEs) that are, on average, closed for more than 50% of the time. This observation illustrates the importance of the open mouth phase for these and other zoobenthic species with marine larval phases.’

Water levels in the Lakes are managed cooperatively by SANParks and during extreme weather conditions in conjunction with Eden District Municipality and the Knysna Municipality as low-lying residential properties adjacent to the estuary often get flooded.

When is the mouth opened?

The method used for this is ‘perodic artificial breaching’ of the estuary mouth. The set standards for breaching are when water levels in the estuary reach 2 meters above mean sea level (amsl) for Swartvlei and between 2,1 and 2,4 meters for the Touw River.

Breaching is done by SANParks, the custodians of the system. Changes in water exchanges could influence the physical and chemical properties of water.

SANParks scientists have measured the temperature of the system closely, salinity, total dissolved solids, dissolved oxygen and others since 1991.

Article by Nandi Mgwadlamba - SANParks.

Picture: Wiki Village.


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