In tidal rivers, spatiotemporal changes in flow conditions that occur during sampling can lead to strong biases if synopticity in the measurements is assumed. A strategy of time-space interpolation between successive crossings of a river section is presented here, to transform non-synoptic data into continuous hydrodynamic fields. Results show that interpolation methods need to be chosen carefully and adapted to the spatiotemporal distribution of sampled points as well as to the underlying physics of the interpolated fields. The proposed interpolation procedure helps deriving the time evolution of depth-averaged velocity and water surface profiles at cross-sections of the St. Lawrence fluvial estuary from non-synoptic ADCP and RTK GPS measurements.