Hydropeaking due to operations of storage hydropower plants was highlighted as one of the main stresses on aquatic ecology by the European Water Framework Directive. Nevertheless, hydropower must be seen as a high-value resource in the renewable energy sector. However, very few studies deal with the physical processes behind the artificial unsteady flows, as the variability of local river morphology and roughness in general. Thus, the aim of the present study is to address these issues on the basis of one-dimensional hydrodynamic numerical modelling. In total, four Austrian rivers have been investigated, which were further divided into 13 sub-reaches according to their bed slope. From single and multiple linear regression models, it was possible to relate potential retentions of peak flow for peak duration of <2 h and for bed slopes <0.005 to the spatial extent of wetted width between base and peak flows, independent of the selected ramping rate (discharge relationship between base and peak flows). Moreover, the ramping velocity (rate of decreasing water level between peak and base flows) correlated positively with an increase in depth variance during peak flow, which was related further to an increase in mean flow velocities and a higher specific discharge over the 13 selected reaches. Nevertheless, within sensitivity testing, it could be proven that increases in channel roughness cause an increase in peak flow retention and a decrease of the ramping velocities for the decreasing limb. Copyright © 2012 John Wiley & Sons, Ltd.