This dissertation concerns the hydrography of hydrothermal plumes and the
problem of measuring the flux of heat from submarine volcanoes to the deep sea.
It presents the results of an experiment designed to measure the heat flux from
the Main Endeavour hydrotherml vent field (MEF) on the Endeavour segment of the
Juan de Fuca ridge in the deep Northeast Pacific.  Primary data are from an
autonomous underwater vehicle called Autonomous Benthic Explorer (ABE), a
lowered CTD, and 2 current meter moorings.

Chapter~\ref{ch:power} motivates heat flux measurement with questions about
the nature of the subsurface oceanic crust and the influence of hydrothermal
venting on the oceans.  Context for the Flow Mow experiment is then established
through definition of terminology, a review of historical heat flux
measurements, and a re-examination of plume theory. 

Chapter~\ref{ch:flowmow} presents the field program and experimental setting,
emphasizing currents and hydrography near the MEF.  A key observation is that
axial currents change dramatically near the depth of the ridge crests --- from
rectified, oscillatory flow within the axial valley to relatively fast, rotary
flow in the few hundred meters above the ridge crests.

The remainder of the thesis is devoted to the problem of estimating flux in the
distinct environments: the axial valley (Chapter~\ref{ch:lower}) and above
the ridge (Chapter~\ref{ch:upper}).  In both parts, a ``puff'' model of plume
advection and diffusion is utilized to understand the field observations, as
well as to predict the patterns of variability that can be generated by
combinations of oscillatory and mean flow.