Hydraulic conditions in isothermal lakes

SUMMARY. The paper considers the forms of water movement that occur in unstratified lakes. Although not dealing directly with ecological implications, it is hoped that it will encourage the inclusion of observations of underwater weather as part of normal limnological routine. It is particularly concerned to show what observations are most effective in revealing hydraulic conditions. Relations between wind, lake geometry and wave characteristics are examined. Factors determining the steady, wind‐driven circulation are reviewed and examined in sequence: the action of wind on a water surface and the current directly generated by the wind drag; the effect of the earth's rotation; the need to balance the flow in and out of any part of the lake; the influence of lake morphometry. A simple model of lake circulation is developed that may help in the interpretation of on‐lake observations. Some other features of isothermal motion are discussed including the horizontal currents associated with unsteady wind conditions (seiches). The possible interaction between waves and currents are discussed. A tentative classification of hydraulic conditions in terms of wind speed, lake depth and fetch is suggested as a basis for forming hypotheses about the motion prior to on‐lake observations and as a possible starting point for a comparative approach.