About 120 km of open concrete channels, locally known as catchwaters, have been constructed across hillsides in Hong Kong to divert surface runoff into reservoirs for local water supply. With cross-sectional areas up to 20 m2, catchwaters pose significant hazards when uncontrolled flows occur as a result of either breaching, or blockage and overtopping beyond the limits of designed overflow weirs. Blockage can also lead to erosive flows along streamcourses directly below the weirs. Different modes of ground failure associated with catchwater breaching or blockage are illustrated. A study carried out in the early 1980s used simplified engineering geomorphological mapping and hydrological analysis of contributing catchment areas (CCAs) to enhance an existing catchwater ranking system and to make a broad assessment of catchwater risk to potential downslope affected areas (DAAs). The mapping involved aerial photograph interpretation (API) to identify CCAs and DAAs, and field walkovers to identify and classify previous failures in man-made slopes adjacent to the channels, and hydraulic defects within the channels. As a result of the study, six catchwaters were accorded priority for geotechnical and hydraulic improvement works. Further studies and improvement works to these and other catchwaters have been carried out in the last 25 years but risks remain, as indicated by several recent incidents. A new phase of integrated studies of the catchwater system is under way. The mapping concepts used in the early study may be of value in future risk assessments.