Many Alpine rivers are affected by hydro-peaking, strong sub-daily fluctuations of discharge caused by intermittent power production from hydropower plants. Adding a retention volume at the outlet of a hydropower plant aims at attenuating hydro-peaking to a level where adverse effects on fish and invertebrates are minimal. The performance of such a retention volume needs to be assessed when extensions to the hydropower system are envisaged to improve operational flexibility. Using a set of scenarios for future power plant operation and a detailed operation model of the retention volume, future performance of a retention basin in Innertkirchen, Switzerland, is evaluated for the planned addition of a storage reservoir to the existing system. This particular retention basin is aiming at reducing the up- and down-surge rates, instead of focussing on limiting the ratio between base flow and peak flow. Three scenarios that assume that hydropower operation is driven by demand are developed, mimicking behaviour ranging from a rather smooth operation to an operation mode with extensive peaking. These scenarios are used in an optimisation model that simulates the operation of the retention volume for each time step based on limited knowledge of future inflows. After the addition of the reservoir, up- and down-surge gradients are expected to comply with threshold values derived from an extensive ecological field study. Increasing the lead time of power plant discharge from 30 minutes to 45 minutes could allow for improved management of the basin.