Human perception of two important visual aspects of the appearance of river and lake water, clarity and colour, was investigated from the perspective of bathing and aesthetics in New Zealand. Bankside interviews were conducted at the same time as measurements were made for clarity and colour. We found that water appearance, perceived suitability of the water for bathing, and bathing activity are closely linked, as is perception of water clarity and colour. Water is perceived as just suitable for bathing at a horizontal black disc visibility of 1.2 m (equivalent Secchi depth c. 1.5 m) with a 90 percentile of 2.2 m (equivalent Secchi depth c. 2.75 m), confirming earlier work. As might be expected, blue waters are preferred to yellow waters, and water is perceived as just suitable for bathing if the Munsell colour is greater than around 30 units (i.e., at the low end of the green-yellow range). For bathing waters, consideration of personal safety is very important; for aesthetics, surroundings are important. People's perception of visual aspects (i.e., clarity and colour) appears to have a strong influence on their rating of overall site suitability.

In a study of human perception of the appearance (clarity and colour) of rivers and lakes for bathing and aesthetic water use, we have shown that: bankside observers can reasonably accurately assess colour (hue) in some circumstances; and that the phenomenon of perceptual set (seeing what is expected to be seen) may influence the visual perception of some waters. Further, perceived naturalness of a water seems to provide for a more positive response towards a water body than might otherwise be expected.

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