Abstract. Turbidity, a measure of water clarity, is monitored for a variety of purposes
including (1) to help determine whether water is safe to drink, (2) to
establish background conditions of lakes and rivers and detect pollution
caused by construction projects and stormwater discharge, (3) to study
sediment transport in rivers and erosion in catchments, (4) to manage
siltation of water reservoirs, and (5) to establish connections with aquatic
biological properties, such as primary production and predator–prey
interactions. Turbidity is typically measured with an optical probe that
detects light scattered from particles in the water. Probes have defined
upper limits of the range of turbidity that they can measure. The general
assumption is that when turbidity exceeds this upper limit, the values of
turbidity will be constant, i.e., the probe is “pegged”; however, this
assumption is not necessarily valid. In rivers with limited variation in the
physical properties of the suspended sediment, at lower suspended-sediment
concentrations, an increase in suspended-sediment concentration will cause
a linear increase in turbidity. When the suspended-sediment concentration in
these rivers is high, turbidity levels can exceed the upper measurement limit
of an optical probe and record a constant “pegged” value. However, at
extremely high suspended-sediment concentrations, optical turbidity probes do
not necessarily stay “pegged” at a constant value. Data from
the Colorado
River in Grand Canyon, Arizona, USA, and a laboratory experiment both
demonstrate that when turbidity exceeds instrument-pegged conditions,
increasing suspended-sediment concentration (and thus increasing turbidity)
may cause optical probes to record decreasing “false” turbidity values that
appear to be within the valid measurement range of the probe. Therefore,
under high-turbidity conditions, other surrogate measurements of turbidity
(e.g., acoustic-attenuation measurements or suspended-sediment samples) are
necessary to correct these low false turbidity measurements and accurately
measure turbidity.