The application of head-flow (hQ) plots for fluvial groundwater flood alert systems is examined using synchronous values of groundwater heads and gauged river flows from instrumented, largely drift-free Chalk catchments in the Thames Valley and South Downs, UK. Three types of head-flow response are identified. These are correlated with the depth to water table. The rapid increase in groundwater discharge per unit rise in water level during periods of more extreme recharge is due to activation of the ephemeral drainage system and shallow zones of high permeability. Annual maximum hQ data tend to overestimate the threshold point at which a catchment becomes more responsive to further recharge. Flood alerts should employ a combination of both head and flow values based on plots of rising groundwater levels during wet winters from monitoring wells located within the ephemeral part of the drainage system.