Measured heat flow in young oceanic crust is generally less than predicted by lithospheric cooling models, even where thick sediments had been expected to isolate the crust from the ocean and hence eliminate hydrothermal heat transfer. At isolated sites with basement outcrops or topographic highs, however, heat flow sometimes exceeds the model predictions. It thus appears that water migrates and upwells at distant sites, hence transferring heat laterally. Simple estimates of the lateral flow distance can be made when heat flow data have adequate areal sampling. The data can be parametrized by distance from the presumed upwelling site, and integrated to estimate the area from which heat must be transferred. Along the Juan de Fuca ridge, the minimum effective lateral flow distances are 8 km for 3--5 Ma crust, and 2 km for younger (<1 Ma) crust.¿ 1997 American Geophysical Union