Internal tidal bores generated by breaking internal waves cause dramatic, high-frequency variation in temperature, salinity, water velocities, and concentration of chlorophyll a on Conch Reef, Florida Keys. The arrival of bores on the reef slope is linked to a semidiurnal internal tide and is marked by temperature drops of up to 5.4°C and salinity increases of up to 0.6‰ in 1–20 min. These changes are accompanied by the sudden onset of upslope flow 1–15 m above the bottom with speeds of 10–30 cm s−1. Cool, high-salinity water is transported from below the thermocline seaward of the reef and is resident on the reef slope for up to 4 h before it mixes with surface waters and recedes downslope. Compared with ambient surface water, this deep water can contain significantly elevated concentrations of dissolved nitrate. Physical variability produced by this mechanism increases significantly with depth on the reef slope. Analysis of 3-yr temperature records indicates the arrival of internal bores is a consistent feature at this site from May through November, with peak activity in July–September. Pulsed delivery of subthermocline water appears to significantly affect the temperature, nutrient, and particle flux regimes on this coral reef.