The effects of a single freshwater release into the Kromme estuary Part 5: Overview and interpretation for the future

Abstract

The Mpofu Dam completely blocks the Kromme River a few kilometers above the estuary and prevents a normal flow of freshwater. Because of this, the biological characteristics of the estuary have changed and the system is now more like an arm of the sea than a productive estuary. An annual allocation of 2 x 106 m3 water (less than 2% of the natural mean annual run-off) is released to prevent the salinity in the estuary from rising above that of the sea (i.e. 35‰). An experiment was undertaken in which the annual allocation of freshwater was released as a single pulse in an attempt to create a salinity gradient and stimulate a biological response particularly in the estuary water column. The release was expected to create freshwater conditions throughout the upper half of the estuary. Instead the low density freshwater flowed over the more dense seawater and there was little mixing. A week after the release both the vertical and longitudinal salinity gradients began to disappear.

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