Transparency

High transparency means you can see the disk when it is deep under the water.

Transparency (sometimes called clarity), like turbidity, is a measure of how far light can travel in water. Although the suspended particles that influence the turbidity reading can include organic particles (microbes, algae and plant particles, and animal detritus) as well as inorganic particles (silt and clay particles), the transparency of the Red River water is usually a measure of the relative concentration of the inorganic particles that account for most of the total suspended solids (TSS). The fewer particles suspended in a sample of water, the easier it is for light to travel and the higher the water's transparency, or clarity.

Measuring Transparency
Transparency is measured using a Secchi disk or a transparency tube (a plastic tube with a Secchi disk pattern at its base). Transparency is the distance (in centimeters) through the water from which a person can clearly see the pattern on a Secchi disk. The transparency can be given as centimeters of depth (Secchi disk) or height (transparency tube). The lower the concentration of suspended particles (or dissolved materials like dark-colored humic acids, common in peat bog water), the greater the distance light can penetrate through the column of water and the greater the transparency.

Transparency in the Red River
Transparency varies seasonally for the Red River. In the winter, when the ground is frozen, precipitation is stored in the snowpack, and runoff and erosion are very low, the transparency increases (from 20 to more than 80 centimeters, or from 8 to more than 32 inches). During the summer, however, the suspended particles carried into the river by runoff and bank erosion reduce transparency to less than 20 centimeters (about 8 inches) and as low as 5 centimeters (2 inches).

Graph of transparency (cm) for the Red River in the FM metro area for the period July 2001 to April 2003.