At our study site in the western half of the Namib Desert rainfall is extremely low averaging between 0 and 12 mm a year. Fog events are, however, more frequent, occurring on average 60–200 days per annum. Research has demonstrated that where there are frequent fog events trees have adapted to intercept and utilise the moisture from these events. In hyperarid environments such as at our study site trees are confined to the banks of ephemeral rivers. Here we determine the extent to which these trees are able to utilise both deep groundwater as well as fog water. Keywords: ephemeral rivers, δ18O/δ2 H isotopes, IsoSource, Namibia, riparian trees, water use.