Bot River Estuary – Should we interfere?

By Rod Bally and George Branch
(Article Source: African Wildlife, Volume 40, No. 6)
This article is also available as a PDF download

Page 1: Introduction
Page 2: Heated Debate
Page 3: How the bot River Estuary Functions as a Physical System
Page 4: The Fauna and Flora and Their Ecology
Page 5: Artificial Breaching
Page 6: Further Reading

Of all the ecosystems in southern Africa, none is more threatened than our estuaries.    They are small in extent, extensively utilised by man, influenced by events that occur anywhere in their catchments and depend vitally on a regular supply of fresh water and contact with the sea.

In recent years, the Bot river estuary (also known as the “Botriviervlei”) has been the subject of a research programme focusing on the question of how the estuary should be managed.  The “Bot” is one of the largest estuaries in the Cape Province and lies only 100 kilometres from Cape Town.  Nowadays, it is used mostly for recreation (fishing, sailing, windsurfing and bird-watching) but in the past it was also used for commercial fishing and even as a landing site for military flying boats during the Second World War.   It is normally a closed estuary, having a sandbar that blocks and dams its mouth.   Each year the water level in the estuary rises when the winter rains descend and flooding of the banks occurs frequently.   Low-lying structures, such as the cottages near the mouth and the Yacht Club’s boathouse, are threatened almost annually.  In addition, the rains dilute the estuary, slowly lowering its salinity until, at times, it becomes so low that massive mortalities of fish occur.  Because of this, the mouth has been bulldozed open every few years to allow the fresh water to escape and to replenish the seawater content and fish stocks.

Traditionally the mouth has been opened at Sonesta (the present “Meerensee” resort) because the dunes there are narrow and made of soft, loose sand.  The outflowing flood of water carves a deep, wide mouth, allowing maximal loss of fresh water and accumulated sediments and substantial penetration of sea-water into the estuary.  Consequently, the mouth remains open for a relatively long period.

Unfortunately, the act of opening the mouth results in a rapid drop in the water level of the vlei and a sudden change in salinity, killing the extensive weed-beds and leaving unsightly and foul-smelling mudflats covered by dying weeds and invertebrates which rot in the sun.  Within a matter of months, the mouth closes again but it takes a few years before the estuary is completely full once more.