Estuarine flow in Juan de Fuca Strait is highly seasonal in nature and an appreciable along-channel surface pressure gradient is coincident with the peak Fraser River discharge. An upper layer momentum balance requires the vertical eddy viscosity Av to be of the order of 0.02 m2 s-1, slightly larger than implied by conventional empirical formulae and used in current models. Mixing in an estuarine channel and a large vertical eddy viscosity should lead to significant cross-channel flows. This secondary circulation, which may resemble internal Ekman layers and intrusions of water mixed at the sloping sides of the strait, is much less understood than the basic along-channel estuarine exchange. Unfortunately, historical current meter data in Juan de Fuca Strait are of insufficient spatial resolution to show the predicted patterns. ¿ 1998 American Geophysical Union