In spite of the undoubted aesthetic importance of water clarity, apparently no studies of human detectability of clarity changes have been made. In this study, finely divided limestone (calcite) was injected into a small natural stream (flow=349 l/s) and the resulting changes in water appearance were monitored by panels of observers while the in-stream concentrations were monitored instrumentally. Initially the stream bed (gravel with some sand) was clearly visible at depths of 760 mm or less through water of 1.5 m visual clarity (horizontal sighting range of a black disc). The cue to increased turbidity is probably partial obscuration or "fogging out" of bed features. Median detection thresholds (just noticeable differences) in terms of reduction in visual clarity below background (initially with the bed clearly visible) were 12 and 10% for two independent panels (each with n=11) who observed the visual changes as turbidity was increased monotonically. This low detection threshold represents a remarkably high human sensitivity to turbidity change that may be generally applicable to small, clear streams. (auths)