Water clarity is perhaps the most obvious feature of surface waters and is the characteristic most people use to make a preliminary judgment about water body health. While it’s true that a lack of clarity can adversely affect the “designated uses” of a water body, such as swimming and other recreational activities, streams with low water clarity are not necessarily “unhealthy” or polluted. Even crystal clear water can contain toxic substances, and while water bodies with low water clarity may be undesirable for contact recreation, that does not necessarily mean they are poorly suited for other activities such as fishing. For more information read the “Learn Mores” Trophic State Index (TSI, pertains to lakes and estuaries), Water Quality Index (WQI, pertains to streams) and Impaired Waters.
What causes poor water clarity? Erosion in the watershed can cause particles of soil and organic matter to become suspended in the water. Boat traffic can stir up bottom sediments and cause turbidity. Nutrient pollution can cause overgrowth of algae, both on the surface and in the water column. All of these factors are related to human activity and controlling them can greatly improve water quality.
There is an important difference between “true color”, which is caused by substances that are dissolved in water, and “apparent color”, which is caused by substances that are suspended in water, such as plankton, sediment, and leaf fragments. Apparent color is temporary, and often seasonally-dependent and flow-dependent, whereas true color is often a prevalent feature of the waterbody year-round.
Some waterbodies are referred to as “black waters” and are tea-colored or coffee-colored. Do not let their color deceive you; many of these waterbodies are among the healthiest in the country. Their color is mostly the result of humic acids (tannins1
) that leach from living plants and dead plant matter that are either in the water or in low-lying areas like marshes and swamps that surround the waterbody.
1tannin (n.) – a yellowish or brownish bitter-tasting organic substance present in some galls, barks, and other plant tissues, consisting of derivatives of gallic acid
2lignin (n.) – a complex organic polymer deposited in the cell walls of many plants, making them rigid and woody