Environmental flows are one of the tools being used to help counteract the impacts of river regulation on hydrology, the hydraulic environment and river ecosystems. There is an increasing interest in the delivery of environmental flows that are wholly contained within the channel below bankfull level. Inundation of riverbanks and other in-channel geomorphological features can increase river productivity and create slackwaters (shallow areas having low velocities) that are important for the survival and recruitment of aquatic organisms. Two-dimensional hydraulic models were used to examine the relationship between river discharge and extent of wetted benthic surface area and the area of slackwater in reaches of four rivers in the Edward-Wakool system in south-eastern Australia. We examined the change in wetted benthic surface area and the change in area of slackwater (<0.02 ms^-1) and slow flowing water (0.02 - 0.3 ms^-1) for six discharge scenarios ranging from low flows to bankfull flows. The relationship between discharge and wetted surface area for these four study rivers was not linear and was strongly influenced by geomorphology. This strongly suggests that the future planning and delivery of in-channel environmental flows should not rely entirely on models that are based on discharge alone. In-channel 2D hydraulic models can be used to i) facilitate better planning and management of in-channel environmental flows, ii) predict the outcome of in-channel flows on ecosystem functions and biota, iii) assist the interpretation of monitoring data, and iv) assist the communication of outcomes of environmental flows.