The formation and evolution of a tidally modulated river plume that spreads and mixes into English Channel waters is investigated by combining results from two observational studies. Remote sensing imagery showed a near-semicircular plume expansion as its diameter increased linearly during the ebb tide. Subsequent ship surveys made repeated transects across the plume front and showed that regions of mixing and instability (Froude numbers > 1) were confined to horizontal length scales 1, suggesting that a plume cannot form.