This paper presents a novel technique to quantify in situ hydrodynamic roughness of submerged floodplain vegetation: 3D float tracking. This method uses a custom-built floating tripod that is released on the inundated floodplain and tracked from shore by a robotic total station. Simultaneously, an acoustic Doppler current profiler (ADCP) collects flow velocity profiles and water depth data. Roughness values are derived from two methods based on (1) run-averaged values of water depth, slope and flow velocity to compute the roughness based on the Chézy equation, assuming uniform flow, (2) the equation for one-dimensional free surface flow in a moving window. A sensitivity analysis using synthetic data proved that the median value of the roughness, derived using method 2, is independent of (1) the noise in water levels, up to 9 mm, (2) bottom surface slope, and (3) topographic undulations. The window size should be at least 40 m for a typical lowland river setup. Field measurements were carried out on two floodplain sections with an average vegetation height of 0·030 (Arnhem) and 0·043 m (Dreumel). Method 1 resulted in a Nikuradse roughness length of 0·08 m for both locations. Method 2 gave 0·12 m for Arnhem and 0·19 m for Dreumel. In Arnhem, a spatial pattern of roughness values was present, which might be related to fractional vegetation cover or vegetation density during the flood peak. 3D float tracking proved a flexible and detailed method for roughness determination in the absence of waves, and provided an unrestricted view from shore. Copyright © 2008 John Wiley & Sons, Ltd.