Existing methods of measuring flow velocities in natural rivers are largely based on series of point measurements. Acquisition of these data can be time consuming and difficult, especially in high flow conditions. This paper introduces the use of GPS drifters (termed GRiFTers) to measure surface flow velocities in a 400 m reach of the River Swale, UK. Over 10 000 measurements were made in a 3 hour period and aggregated over a 2 m grid to generate a genuine distributed representation of flow across the reach. The technique shows great promise to provide new insights into flow patterns over long reaches of rivers, over a range of flow conditions, and may also provide valuable data for numerical model validation.Water surface velocity is calculated for two perpendicular horizontal components at time t (u Nt and u Et ), which are then used to calculate the resultant velocity (v t ): R. J. Stockdale et al.Figure 3. Velocities measured over the study reach of the River Swale, UK. The results are averaged over a 2 m grid; the inset shows detail and vectors from one riffle section. This figure is available in colour online at www.interscience.wiley.com/journal/espl 1320 R. J. Stockdale et al.Figure 4. Measurements per grid cell (a) and standard errors in the northing (b) and westing directions (c).