Laboratory measurements have been made of the spectral reflectance of size-fractionated suspended sediments from two estuarine systems. The results show that reflectance at wavelengths between 400 and 1000 nm increases with decreasing particle size for given sediment masses. Preliminary interpretation suggests that a new factor, target area, which is derived from the volume and number distribution of particles in a suspension, is proportional to the reflectance of the suspension. Reflectance in the near infra-red is shown to be unaffected by other in-water parameters and is also largely independent of the colour or nature of the particles. The relationship between spectral reflectance in the near infra-red and target area therefore provides a means of estimating suspended sediment concentration from remotely sensed imagery if the particle size distribution is known or can be estimated.

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