A large floodplain of the River Findhorn, northeast Scotland, was investigated using hydrogeological and hydrochemical methods (including residence time indicators) to characterise groundwater/surface-water coupling and groundwater flooding. The study demonstrated widespread stratification within the floodplain: shallow (<8 mbgl) deposits are highly permeable (100 m/d); deeper deposits have low permeability (1 m/d) and limit interaction with the underlying sandstone aquifer. Hydrochemistry and groundwater-level variations show floodplain groundwater is recharged from the river, surrounding hillslopes and direct rainfall infiltration. The river loses water to groundwater as it enters the floodplain; further downstream, groundwater response follows closely river stage giving rise to complex exchanges; near the sea, groundwater continually discharges to rivers, tributaries and ditches. Groundwater flow is largely parallel to the river and mean groundwater residence times vary from 3 years to 20 years. Groundwater at the edge of the floodplain, close to the hillslopes, has distinctive chemistry and responds rapidly to local intense rainfall (daily total >30 mm). Persistent groundwater flooding occurs within topographical lows and also in the discharge zone where it is largely managed with a series of drains constructed in the 19th century. The significant and complex role of groundwater in floodplains, demonstrated by this study, highlights the importance of fully considering groundwater in flood management schemes.