We present a simple image analysis technique for measuring the sediment composition of the bed surface during flow with bedload transport in flume experiments. The bed was composed by a bimodal mixture of natural sediments and the flow conditions were such that both fractions were mobile. A camera placed 2.5 meters above the flume framed the central part and took pictures every minute, allowing for a high-resolution data acquisition. The two sediment fractions had different colors, thus we developed an image analysis technique based on color detection. This technique provided the areal fraction content of each fraction and the detection of patterns of sorted sediments, namely patches of each grain size, and it was applied in three laboratory experiments conducted to study the influence of lateral width constraint on channel morphology, sediment sorting, and bedload transport. The three experiments were carried out under constant feeding rate conditions, in particular a sediment supply equal to the transport capacity was provided, thereby matching dynamic equilibrium conditions. The image processing was implemented through a code written in Matlab and made available for other users.