south river monitoring

Water Clarity is the measurement of how far sunlight can pass through the water column. Sunlight is one of the key elements needed for underwater grasses to grow. When the water column's clarity is reduced, the underwater grasses do not receive the sunlight necessary for growth. Excess sediment is the leading factor for the South River's poor health.

Because of their small size, the particles of sand, silt and clay that we call “sediment” often float through the water rather than settling to the bottom, and can be carried long distances during rainstorms. When there are too many sediment particles suspended in the water, the water becomes cloudy and muddy-looking. Cloudy water does not allow sunlight to reach the plants that grow on the bottom of the Bay’s shallows. Without sunlight, these plants—including underwater grasses—die, which affects the young fish and shellfish that depend on them for shelter.

Impacts of Excessive Sedimentflatcreek sediment plume

  • Nutrients and chemical contaminants can bind with sediment, spreading through the Bay and its waterways with particles of sand, silt and clay. Fish and shellfish that live and feed on or near contaminated sediment can become contaminated themselves, triggering fish consumption advisories in various portions of the watershed.
  • Excess sediment can smother oysters and other bottom-dwelling species.
  • Accumulating sediment can clog ports and channels, affecting commercial shipping and recreational boating.

How Do We Measure Water Clarity?

Despite advances in science, researchers still use a technique for tidal waters that is hundreds of years old to measure clarity: a secchi disk. This black and white sectioned disk is lowered into the water until the user can no longer see the division between the colors. The disk is then slowly raised until the user begins to make out the distinct sections again. This depth is used as a measure of the transparency of the water. The penetration of sunlight is crucial for the growth of underwater grasses, which acts as a filter, dissolved oxygen producer, and a vital nursery habitat for countless aquatic species. Scientists predict that underwater grasses can survive when the water clarity averages 1.0 meters during the growing season.

For freshwater streams, measuring clarity is more complicated. The Federation traditionally used a conductivity probe on a hand held YSI sonde that is placed in the stream. However, conductivity doesn't directly measure clarity, but rather the electrical conductivity in a solution to monitor the amount of nutrients, salts or impurities in the water. Thus, if there is road salt dissolved in the stream, the stream may appear clear, but still have a high conductivity reading. Recently, the Federation has invested in a turbidity probe, which does measure the amount of suspended particles by measuring how the light is refracted, but the particles could be sediment, algae, bacteria or other. Both of these methods only provide the percentage of contaminates in the water. Without knowing the volume of water going past the probe, you cannot get the total amount of contaminates or load. With all our modern science, we do not know the amount or sediment pouring into the South River within an order of magnitude. We can only tell whether it is improving or worsening.  Yet sediment remains our largest source of pollution for the South River, both by volume and impact.


Historic Water Clarity


2004-2012 Water Clarity (m)

2004-2012 water clarity

 What does this mean?

Water clarity is a good general measure of water quality. When visibility is regularly less than 1 meter, as it was throughout the river up to 2011, that means that the suspended sediment and algae levels, which are driven by nutrient pollution, were so high that sunlight cannot penetrate deeply enough into the water to allow underwater grasses to get established or survive.