Grain size changes in a river dominated by mixed sediment are the outcome of sedimentary grain sizeselective processes. Progress in the measurement techniques that define the spatial and temporal variation in grain size is necessary to provide new insights in this field. Techniques as image analysis and acoustic methods have emerged in the last years as rapid, reliable and repeatable methods in field and laboratory studies. Three laboratory techniques were evaluated in this project for application in later flume experiments: (1) particle coloring in combination with photogrammetric analysis (providing data on the grain size distribution of the bed surface); (2) core sampling combined with threedimensional imaging (providing data on the grain size distribution of the bed surface and substrate); (3) ultrasonic imaging (providing a classification of the bed surface sediment).