Turbidity

High turbidity means you cannot see the disk very far below the surface of the water

Turbidity, like transparency, is a measure of water clarity (how far light can travel through water). The more particles suspended in a sample of water, the more difficult it is for light to travel through it and the higher the water's turbidity, or murkiness. Although the suspended particles that reduce clarity can include organic particles (microbes, algae and plant particles, and animal detritus) as well as inorganic particles (silt and clay particles), turbidity in the Red River is usually a measure of the inorganic particles that account for most of the total suspended solids (TSS).

Measuring Turbidity
Turbidity is measured by an instrument called a nephelometer and is reported in nephelometric units (NTUs). The nephelometer, also called a turbidimeter, has a photocell similar to the ones used in cameras to indicate when the "flash" is needed. The water sample is placed in a column in the instrument, and the photocell reads the intensity of a beam of light showing through the water column. The cloudier or more turbid the water, the lower the intensity of the light that reaches the photocell and the higher the NTUs. High TSS or the presence of dark-colored humic acids from the decay of vegetation, common in the water of peat bogs, would result in a high turbidity reading.

Turbidity in the Red River
Turbidity varies seasonally for the Red River. In the winter, when the soil is frozen and precipitation is stored in the snowpack, runoff and erosion are very low, resulting in low turbidity (less than 10 NTUs). During the summer, the soil and plant particles entering the river from runoff and bank erosion result in high turbidity (above 60 NTUs), with the highest levels in July and August (over 100 NTUs). The local water treatment plants bring the turbidity level down to meet the U.S. Environmental Protection Agency National Primary Drinking Water Standards (turbidity in local municipal water is usually less than 1 NTU).

Graph of turbidity (NTU) for the Red River in the Fargo-Moorhead metro area for the period July 2001 to April 2003.