Estuaries have long been recognized as sites of major compositional fluctuations that can have a range of effects including mineral growth in the water column and in the underlying estuarine sediment. We have studied chloride and sodium concentrations and stable isotopes (H and O) from a suite of estuarine waters from the temperate Anll ons estuary in NW Spain to assess the complexity of estuarine mixing process. Water samples were collected hourly from in estuary sites over three consecutive days and end-member samples were collected on each sampling day. Evaporative concentration of the estuarine waters is demonstrated by the maximum concentration of geochemically conservative chloride being as much as 17.5% greater in the estuary than in the local seawater. Relative to chloride concentration, both d 2 H and d 18 O values of estuary waters tend to be more enriched than would be expected for the simple physical mixing of river water and seawater. These patterns can only be plausibly accounted for through evaporation affecting the water, concentrating the chloride and causing isotope fractionation. We have modelled the effects of evaporation on chloride concentration and the stable isotopes in the estuary; the results closely match the distribution of analytical data suggesting that up to 40% evaporation has happened. We have demonstrated that two thirds of estuarine water samples analysed underwent between 5% and 40% evaporation. This previously unreported degree of evaporation in estuaries has important implications for any processes that is effected by the consequent degree of elevated solute concentration.Estuaries are locations of complex water and sediment mixing patterns due to the twice-daily influx of seawater (Berner and Berner 2012). Estuarine sediments and the signatures that result from the competing influences of fluvial and marine systems are of interest in their own right and as analogues to understand ancient, deeply buried estuarine sandstones. Daily variations in water chemistry through tidal cycles result in complex systems of water mixing, and sediment deposition and resuspension (Pritchard

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