Flooding and erosion are major hazards in semi-arid areas. As a result of climate and/or land use changes these hazards could increase. Vulnerable areas include the Mediterranean region of southern Europe. As part of the European Union (EU)-funded MEDALUS (Mediterranean Desertification and Land Use) research programme on impacts of desertification a model to simulate channel changes is being developed. This is designed to simulate morphological changes in the channels and floodplains of ephemerally flowing streams over time scales of a year to decades. It incorporates morphological, sedimentological and vegetation characteristics and simulates the effects of flood events and of climate and human activities for reaches of valley floor. In this paper the selection of characteristics and processes in the vegetation component of the model is discussed. The effects of vegetation upon flow, erosion and deposition processes and the feedback of flow, sediment and moisture on vegetation are identified. The conceptualisation of the model is outlined then the problems and strategies for parameterisation are indicated.