The distributions of flow and bedload in channel diversions are relevant for numerous practical applications in hydraulic engineering, including the design of structures for hydropower generation, irrigation, drinking water supply, cooling water discharge, flood diversion into side channels, and fish passage facilities. Due to the Bulle effect, bedload, which follows the secondary flow patterns near the bed, is disproportionately directed into the side channel. In addition to the well-established influence of the flow distribution, the cross-sectional aspect ratio (the ratio of bed width (b) to flow depth (h)) has recently been identified as a key parameter affecting bedload distribution at channel diversions. This study experimentally analysed the effect of the cross-sectional aspect ratio superimposed on the discharge distribution under non-fully mobile bed conditions. Laboratory experiments were conducted in a flume featuring an acute-angled diversion with a rectangular cross-section, under fully turbulent and fully subcritical flow conditions, using sediments with significantly varying specific densities. The results indicated that the bedload distribution in the straight channel increased with a higher b/h value, a relationship incorporated into a new analytical approach. Furthermore, comparisons between different bedload materials (sand and lightweight granulate) revealed similar deposition patterns when the discharge distribution and b/h were held constant. In addition, measures to reduce bedload entry into the side channel were analysed. The findings are particularly significant for the design and operation of water withdrawal from open channels with bedload transport.