Temporal lag effects, which are viewed as the inability of an alluvial system to immediately respond to changing flow conditions, are investigated under bed load sediment transport conditions. Temporal lag effects are investigated experimentally and a temporal lag model is formulated. The temporal response ofthe alluvial system is characterised by a single variable, the equivalent steady flow rate. An impulse model is used to develop a general temporal solution. The performance of the temporal lag model is tested against measured data and is verified. A numerical model which incorporates the temporal lag model and its performance are also briefly described.