The paper reviews the generic characteristics of ephemeral rivers compared with seasonal and perennial systems in the context of southern Africa. The difficulties of estimating the natural characteristics of ephemeral systems are discussed as well as the difficulties of estimating the impacts of water resource management strategies. Many of the problems are associated with the potentially discontinuous occurrence of flow in both time and space and the fact that static pools, as well as flowing water, may be of ecological importance. The most obvious impact of water resource developments on ephemeral rivers may be a reduction in the number and size of flow events. However, delays in the onset of flow in seasonal rivers and changes in the duration, quantity and quality of in-channel pool storage may be of equal importance. In a number of South African ephemeral channels the major impacts have been caused by the importation of water from elsewhere and a consequent reduction in streamflow variability. The paper suggests that there are a number of gaps in the understanding of the hydrology of ephemeral rivers, but perhaps more importantly, in the methods that are available to supply the type of information required by ecological specialists to be able to determine ecological water requirements under managed flow situations. Copyright © 2005 John Wiley & Sons, Ltd.