An array of temperature and velocity sensors was deployed for two weeks in the bottom boundary layer at the Southern Juan de Fuca Ridge during the National Oceanic and Atmospheric Administration VENTS'90 field experiment. The instruments were placed close to the seafloor to detect effluent from diffuse hydrothermal sources located 1--2 km distant. Over the duration of the field deployment, we detected effluent from two vent sources. The plume from the larger of the two sources was sensed at all of the instrument locations, permitting estimates of dilution based on the field measurements. Our data also permitted estimation of energy flux from diffuse vents (approximately 125¿75 MW from a source to the north of the instrument array, and 12¿8 MW from a small source to the east). The largest source of error in the data was estimation of the plume width. We have described a sensor array that would reduce this error in future work. Such an array would be suitable for a long-term vent activity monitor. ¿ American Geophysical Union 1994