Suspended sediment properties and visual clarity of the Manawatū River, New Zealand

Abstract

ABSTRACT Studies of river suspended sediment typically focus on mass loads with little attention devoted to sediment properties such as particle size distribution (PSD) or light attenuation. However, environmental behaviour and ecological effects of suspended sediment vary greatly with particle properties. Visual clarity is a particularly important attribute of waters that is strongly affected by particle properties. We studied the relationship between visual clarity and suspended sediment in the Manawatū River – by combining event auto-sampling with long-term state-of-environment monitoring. We fractionated stormflow samples into sand and ‘mud’ (silt + clay) by wet-sieving at 63 μm, and the mud fraction was further analysed for organics, PSD and light beam attenuation (which controls visual clarity). Light beam attenuation correlated very closely with high-frequency field turbidity – providing an absolute optical calibration. The ratio of light beam attenuation to mass concentration (known as ‘optical cross-section’) averaged about 0.5 m2/g – somewhat higher than expected for silt particles in the measured size range, probably due to plate-shaped clays. The visual clarity regime, estimated from field turbidity, was consistent with monthly direct observations. Our study illustrates how certain sediment ‘quality’ metrics can usefully extend sediment monitoring applications, and we encourage closer integration of sediment and water quality monitoring.