Temperature and suspended particulate matter concentrations are used to identify lake‐river interactions in Lake Wakatipu. Results from sampling at seasonal maxima and minima of water temperature and inflow, in conjunction with density estimates, suggest that both annual and diurnal cycles from inflow from the predominant headwaters are generated. From mid autumn to mid spring, underflows predominate; in summer, river water may warm sufficiently to allow inflowing water to interflow or overflow. In summer the large diurnal temperature range in the rivers sets up a diurnal cycle of inflows, with underflow in the early morning, progressing through to interflows and finally overflows in the middle of the day, before returning to underflows again at night. In winter, when river temperatures remain low throughout the day, inflowing water underflows continuously. Floods normally enter as turbid underflows, disrupting the diurnal summer cycle and strengthening the underflows of winter. Flood underflows reach speeds of over 20cm s‐1and carry particulate material over 60 km downslope into the deepest basin.