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Some aspects of the optical turbidity of British Columbia inlet waters Giovando, Laurence Frank

Abstract

A light-scattering method has been utilized to determine the seasonal and geographical variation of optical turbidity in the waters of the major inlets of the southern British Columbia coast. (The optical turbidity is here defined as the fractional decrease in light intensity per meter due to the presence of suspended material in the water.) The major contribution to the turbidity in the inlets is the minerogenic material brought into the inlet by rivers at or near the head. The inlets whose rivers are primarily glacier-fed possess the highest turbidity values and exhibit the most marked seasonal variation of turbidity. The net outflow of water in the shallow layers-which is a prominent feature of the circulation in the inlets-is the basic mechanism by which the material introduced by rivers is distributed throughout the length of the inlet. The surface values of turbidity range from about 0.5 to over 30 meters⁻¹ (m⁻¹) in the summer and from about 0.1 to 1 m⁻¹ in the winter. The values decrease from head to mouth, the effect being especially marked in the summer. The main body of water in the inlets usually possesses uniform turbidity at any time of the year, values ranging from 0.1 to 0.7 m⁻¹. A marked increase in turbidity occurs, in the bottom layers of water, in all inlets. In the shallower inlets, this increase appears to be due primarily to tidal scouring of bottom material. In the deep inlets, it is presumably due to two causes: intermittent intrusion of deep water from outside the inlet, and, to a more prominent degree, turbidity currents originating at the inlet head. Evidence suggests that these currents are slow and possess a frequency of occurrence of the order of weeks. The contribution of material of biological origin to the turbidity is confined primarily to the inlets with small runoff. Little or no dissolved coloured matter is present in inlet waters. Size analysis by microscope indicates that the suspended material averages somewhat below 10μ in the major portion of an inlet; average sizes of up to 17μ occur near the head of inlets during large runoff. There is little material below 1μ in size. Light-scattering measurements indicate that the suspended material is preponderantly anisotropic in nature. The concentration of material varies from less than 1 to over 100 parts per million by volume. By means of the turbidity and concentration values obtained, it has been estimated that the rate of sedimentation in the inlets ranges from about 35 cms to about 650 cms per 100 years, the value increasing from mouth to head of the inlet. The following relation between the Secchi disc reading D and the average turbidity T over the distance D has been found: T = 1.8/D(1.2)

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