Effects of dam operation on downstream river morphology: the case of the middle Zambezi River

The sand-bed middle Zambezi River flows between two large dams: the Kariba in Zambia and Zimbabwe upstream and the Cahora Bassa in Mozambique downstream. Since the construction of Kariba for hydropower production in 1959, the river discharge has been regulated according to two different strategies. This makes the middle Zambezi a unique case for studying the effects of different dam operations on the long-term morphological trends of the river. To analyse these effects, this work focused on the interaction between the river morphological changes and the groundwater flow. The comprehensive approach, combining numerical modelling, analysis of field and historical data, as well as theoretical considerations, led to the conclusion that the morphological response of the middle Zambezi to damming has been mainly affected by the sudden closure of the dam flood-gates, originating seepage forces causing bank instability and river widening as a result. The local climatic oscillation, comprising dry and wet periods having a duration of tens of years, was found to have played a major role in the flow regulation, which resulted in a constant discharge in the period between 1982 and 2000.