We characterize hydrothermal plumes and hydrography within the axial valley of the Endeavour segment (Juan de Fuca ridge, northeast Pacific) using data from an autonomous underwater vehicle, a lowered CTD, and two current meter moorings. Our survey of the Main Endeavour hydrothermal vent field (MEF) shows hydrographic variability on scales as short as ~20 m and ~10 min. Within the ~100 m deep axial valley, the mean flow is 1--5 cm/s to the north, and tidal oscillations have ~5 cm/s amplitude. Combining the mean flow with an estimate of the difference in average temperature observed north and south of MEF, we find that the net along-axis horizontal heat flux has a mean magnitude of ~8--42 MW. An advection/diffusion model forced by local current records explains the observed temporal hydrographic variability and shows that mean horizontal flux variance is correspondingly high (standard deviation ~63 MW). A steady state MEF heat budget, constrained by the observed horizontal flux and measurements of vertical fluxes from other studies, has two implications: the MEF heat flux is partitioned about equally between diffuse and focused vents, and about 85% of the flux from diffuse plumes is entrained by focused plumes.