In the framework of a research project studying the influences of pumping/turbine modes on turbulence, flow fields and suspended sediments in reservoirs, prototype measurements are carried out at the Grimsel II pumped storage plant (Switzerland). In situ recordings comprise flow and velocity patterns in the lower reservoir. For the measurement of flow velocities, Acoustic Doppler Current Profiler (ADCP) devices have been fixed on the reservoir bottom in front of the intake/outlet structure. The temporal evolution of three-dimensional velocity profiles in front of the intake is compared to the operation data provided by the hydropower producer. Flow fields corresponding to the expected main direction of the in- and out-flowing jet are observed during both pumping and generating activity. Periods, where their orientation and the expected main direction of the jet do not correspond clearly are studied in more detail, splitting up data series in characteristic sequences and applying frequency analysis. Such signal processing reveals the correlation between the velocity profiles in front of the intake/outlet and the change between pumping and turbine mode.