Water colour and clarity of New Zealand rivers under baseflow conditions

Abstract

Visual clarity and apparent colour (hue) of 96 New Zealand rivers was investigated under baseflow (< median flow) conditions. Visual clarity, measured as the horizontal sighting range of a black disk, ranged 40‐fold from 0.25 to 10.75 m. Only 7% of the clarity observations were “low” (black disk range < 1.2 m), whereas 31% were “high” (> 4 m). Visual clarity was strongly related to nephelometric turbidity (r = ‐0.88) but was less strongly related to suspended solids concentration (r = ‐0.77), even though scattering and absorption of light by suspensoids accounted for most (87 % on average) of overall light beam attenuation. Hue, as described in the field while viewing black disk range, varied from the blues of optically pure water to orange in waters with high concentrations of organic material, particularly yellow substance. There was a weak tendency for clear river waters to be blue‐green in hue whereast turbid waters were more typically yellow‐coloured.