A cross‐shore model of tidal flat heat and mass fluxes is developed to understand the heat exchange between the sediment bed and the water column. A convective heat‐transfer coefficient is used to model sediment‐water heat fluxes which are as great as 20% of the incoming solar shortwave radiation. The model results match well with observations and are used to assess processes across tidal to seasonal time scales. During the summer, tidal flat sediments store incoming shortwave radiation during exposure and act effectively as a net source of heat to the water column. This pattern changes in the winter, when the flats cool during exposure and act effectively as a net sink of heat. Additionally, during the summer water temperatures at the edge of the flooding front are elevated 5°C above the surface sediment temperatures. Model results replicate this process only when water column light extinction coefficients are high, consistent with visual observations of high turbidity (and thus high light absorption) at the leading edge of the flooding