The fact that vigorous hydrothermal discharge occurs on ridge segments with a variety of spreading rates and morphological characteristics makes it difficult to discern the large-scale geological controls on the distribution of hydrothermal venting along the mid-ocean ridge. We undertook to simplify this problem by mapping the distribution of hydrothermally produced thermal and light attenuation anomalies along the crest of the Juan de Fuca Ridge, a medium-rate spreading center with segments of markedly differing morphology. Hydrothermal anomalies were detected by towing a high-precision conductivity-temperature-depth-transmissometer (CTDT) package in a sawtooth pattern through the lowermost 1000 m of the water column above each segment. These surveys, conducted between 1985 and 1989, have produced the first quantitative and continuous visualizations of hydrothermal plumes along a multisegment portion of the mid-ocean ridge. Hydrothermal discharge is strongest on those segments, or portions of segments, where the apparent magmatic budget is highest, as indicated by the degree of along-axis inflation and other morphological characteristics. This correspondence implies that magma is episodically delivered in small and spatially discontinuous bodies of melt rather than by the constant resupply of a steady state magma chamber. We propose that the probability of discharge occurring at any point on the mid-ocean ridge is proportional to the spreading rate. This probability, scaled by the observation that significant plumes presently emanate from ~20% of the Juan de Fuca Ridge, is expected to range from ~5% on the slowest-spreading ridge to ~50% on the fastest-spreading ridge.