Virginia is home to a number of waters commonly referred to as “swamps.” They are characterized by slow moving water, a poorly defined stream channel, and water with low dissolved oxygen (D.O.) and high acidity (low pH). The naturally low D.O. is due to slow-moving, ripple-less waters, where the decay of organic matter depletes D.O. at a faster rate than it can be replenished. Indicators of these conditions include the presence of wetlands, low pH due to organic acids (tannins, humic and fulvic substances) produced in the decay process, and occasionally a brownish-yellow color due to a high concentration of dissolved organic matter.

Waters that are shown to have naturally low D.O. and pH levels are classified as Class VII, Swamp Waters. They have their own corresponding D.O. and pH criteria. These waters are identified in their respective river basin tables of the Water Quality Standards (See sections 9VAC25-260-390 through 540).