Visual Water Clarity and Suspended Substance under Baseflow Conditions in the Nishibetsu River, Japan.

Abstract

Visual water clarity of the Nishibetsu River under baseflow conditions was obtained in 1993 by the black disk method which measures the maximum sighting length by horizontally viewing a black disk in river water. The visual clarity was more than 7 m at the upper reach, but it decreased to less than 2 m at the middle and the lower reaches. A very strong negative correlation was found between visual clarity and suspended substance concentration (SSC) of river water. This relation showed that the visual clarity of clear water decreases remarkably with even a slight increase of SSC, whereas that of turbid water is not affected by such an increase. The ignition-loss analysis indicated that the suspended substance was mainly composed of inorganic matter. The grain size distribution of suspended sediment was similar to that of a fine portion of river bed sediment and river bank sediment. Moreover, the suspended substance flux carried out by all tributaries in this basin was considerably less than that of the main stream. Most of the suspended substance was thus judged to be supplied from the banks and/or the beds of the main stream. The increase-rate of suspended substance flux per unit distance was low at both the upper and lower reaches, while that at the middle reach was high.