Turbidity limits light availability in many glacier-fed lakes and reservoirs, with far-reaching ecological consequences. We use field observations and hydrodynamic modelling to examine the physical processes affecting turbidity in the epilimnion of a glacier-fed hydroelectric reservoir in response to changes in reservoir operations (e.g. water level, inflows and withdrawals), and to natural processes (e.g. particle settling, internal seiching and upwelling). The combination of cold inflows and deep outlets leads to plunging inflows and the isolation of the epilimnion; this isolation, along with particle settling, results in a remarkable clearing of the epilimnion during summer. We simulate a wide range of scenarios based on 46 years of historical flows. We find that the water level and inflow rate in spring control epilimnetic turbidity at the beginning of summer, and this turbidity is a primary determinant of the turbidity and light penetration for the rest of the summer. Turbidity during summer is also impacted by wind-driven thermocline motions. We examine these motions using wave characteristics diagrams and two-dimensional spectra and identify the period and wavelength of the two dominant wave modes: the fundamental internal seiche (\(\approx\) 4 days) and diurnally-forced waves. Occasionally, internal motions are large enough to upwell turbid metalimnetic water to the free surface at the upstream end of the reservoir. These upwelling events coincide with peaks in the inverse of the Wedderburn number. Pulses of upwelled water are advected downstream, setting up a longitudinal turbidity gradient.