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.
The primary causes of reduced water clarity in the South River are suspended sediments and algae. Both the suspended sediments and algae are a result of stormwater runoff causing plumes of sediment and nutrients to be delivered to the South River. There are now sections of the South River between Flat Creek and Rt 50, where the sediment is permanently suspended in the water column.
In the headwaters of the South River, fresher water entering the river flows over the denser saltwater that has intruded from the mouth. As this buoyant plume of fresh water spreads out over the saltier water, the fluid velocity decreases and as a result, the fine particulates that have entered through erosion and stormwater runoff sink into the landward flowing salt layer. The particles are now carried upstream where they are reinjected into the upper flow creating a closed loop. Over time, the concentration builds up in this closed loop area until the concentration that is lost downstream by the ebbing tide equals that which is carried into the area over a tidal cycle. This zone is called the Turbidity Maximum and has the highest concentration of suspended sediments therefore the lowest water clarity.
2004-2012 Water Clarity (m)
2011 Median 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.
Permission must be granted from the Executive Director or the RIVERKEEPER®to use any data and/or graphs