Seasonal and diurnal variations in turbidity in midwestern freshwater streams

Study region: Midwestern United States. Study focus: Turbidity is a commonly used metric of water quality in streams and can be influenced by many factors. The objective of this study was to explore how turbidity changes on short, daily, timescales. New hydrological insights for the region: Turbidity was measured at 15-minute intervals over a seven-year period in two watersheds in central Illinois. We documented diurnal turbidity cycles that were consistent and independent of discharge. Maximum daily levels occurred between 23:00 and 05:00, and minimum values occurred between 13:00 and 18:00, with night-time turbidity averaging 2.3 times higher than day-time turbidity values. The diurnal magnitude of turbidity change for Money and Six Mile Creeks tended to be significantly greater during June, July, August and September relative to other periods of the year. These months coincide with water temperatures above 10°C which suggests a potentially important role of biological activity as a main driver of diurnal turbidity changes. During this warm-water season, the diurnal turbidity magnitude also corresponded with the lunar cycles. Median nighttime turbidity values for warm months during lunar phases with lowest moonlight levels (new moon, waxing and waning crescent) were typically higher than during the phases with highest moonlight availability (full moon, waxing and waning gibbous), which may be related to nocturnal foraging and mating activities of fish, amphibians and bivalve species that inhabit both streams.