The Glengonnar Water has experienced severe environmental degradation resulting from historic lead mining operations. Though now abandoned, the legacy of the mining era remains in the form of river and floodplain sediments grossly polluted with metals, particularly lead which exhibits surface concentrations in excess of 75 000 mg kg−1 total-Pb. Geomorphological investigations link the most recent mining period to a major phase of floodplain aggradation, which acted as an efficient sink for particulate-bound metals. The pattern of metal contamination within the floodplain has been complicated by subsequent fluvial entrenchment. However, preliminary evidence indicates that lateral and vertical variations of metals (both total and available) can be accounted for in terms of age of deposit, distance downstream and local hydraulic conditions. The effectiveness of floodplains to interrupt the delivery of sediment-bound contaminants was highlighted by a metals budget developed for the main channel length. This confirmed that channel-bank erosion of contaminated floodplains soils is now the major source of metals to the fluvial system.