This dissertation discusses hydrothermal plume hydrography and heat
flux measurement at the Main Endeavour hydrothermal vent field (MEF) on the
Endeavour segment (Juan de Fuca ridge, northeast Pacific).  Observations are
from an underwater vehicle called Autonomous Benthic Explorer (ABE), a lowered
CTD, and 2 current meter moorings.

Chapter~\ref{ch:power} contains motivating questions, terminology, plume
theory, and a review of past heat flux measurements at the MEF.  A new
correction factor is derived relating source heat flux to plume heat flux
calculated with isohaline temperature anomalies.

Chapter~\ref{ch:flowmow} presents setting, methodology, and an analysis of
hydrography and currents near the MEF.  Hydrography varies on scales as short
as 10--50\,m and 10--60\,min, and fluid is warmer on average than at the
segment ends.  Oscillatory currents change from multidirectional above the
ridge to rectilinear within the axial valley \cite{thomson+03}.  Northward mean
flow of 2-5\,cm/s within the valley is aligned with the rectilinear
oscillations ($\sim$5\,cm/s amplitude), while southwestward mean flow of
5-10\,cm/s above the ridge is only intermittently aligned with the
multidirectional oscillations ($\sim$5\,cm/s amplitude).  

Heat flux is estimated within the axial valley (Chapter~\ref{ch:lower}) and
above the ridge (Chapter~\ref{ch:upper}).  In both cases, an
advection/diffusion model simulates plume distributions and characterizes heat
flux statistics.  The mean \emph{horizontal} heat flux within the axial valley
through vertical control surfaces 0--100\,m above bottom (mab) is $\sim$76\,MW,
based on warming north relative to south of the MEF and the northward mean
flow.  The modeled standard deviation of this horizontal flux is $\sim$114\,MW.
The \emph{vertical} heat flux in plumes rising through a horizontal surface
$\sim$100\,mab \cite{stahr+03} is 643$\pm$116\,MW.  This vertical flux,
previous source flux measurements, and the horizontal flux together imply that
heat flux partitioning between focused and diffuse sources is $\sim$6:1,
contradicting the prevalent view that diffuse sources account for 90\% of the
heat flux at vent fields.  The net horizontal heat flux \emph{above the ridge}
through vertical control surfaces extending from $\sim$100--400\,mab is
442$\pm$213\,MW, consistent with the vertical flux.  Past estimates of flux in
plumes are higher because they are not net fluxes and likely include
contributions from multiple vent fields.