Lowering Secchi disk to determine relative turbidity (amount of suspended
sediment) in the water. The depth at which the disk is no longer visible can be
related to the amount of suspended material in the water.
Image ID: fizh0137, NOAA's Historic Fisheries Collection Location: Alaska Southeast Photo Date: 1960 circa Photographer: J. Olson BCF Credit: NOAA Central Library Historical Fisheries Collection