An improved method of detecting buoyant hydrothermal plumes and locating th
eir source vents is introduced. Plumes are detected by computing fluid stab
ility from conductivity, temperature, and depth measurements acquired durin
g navigated, towed, vertically oscillating casts over the Endeavour Segment
of the Juan de Fuca Ridge. For each instability detected, the maximum rang
e to its hydrothermal source is estimated by multiplying a theoretical plum
e equilibration time by a measured current velocity. Using an estimate of c
urrent direction, the method reliably locates plume soul-ces where they are
known to exist: in all of the focused vent fields mapped by submersible an
d in several isolated, diffuse flow sites. The method generates a distribut
ion of hydrothermal sources that is more consistent with variations in surf
ace permeability than with circulation cells spaced evenly along a uniforml
y permeable axis. Axial instabilities are nearly continuous along the heavi
ly fissured and fractured western wall of the axial valley. Beyond the axia
l valley, instabilities evidence that hydrothermal upflow penetrates the ou
ter slopes of the ridge crest, probably along a boundary between sheet and
pillow flows.