The chemical process by which elements (e.g., sodium,
calcium, aluminum, etc.) are selectively dissolved out of the solid
substrate of the riverbed or stream bank. Leaching occurs under slightly
acid conditions. When organic matter falls into the river, especially
leaves, leaching also occurs, allowing soluble substances such as sugars,
plant pigments and other organic compounds contained within the leaf
to dissolve into the water. The input of these plant-derived materials
in the fall triggers the growth of aquatic microbes that help prepare
the leaf surface for the shredder species of macro invertebrates. In
regions where acid deposition and precipitation is the rule (i.e., Northeastern
United States), seasonal leaching of heavy metals, primarily aluminum,
into the river causes the death of countless fish and macro invertebrates.
Acid environments also inhibits the growth of aquatic microbes. Coal
and gold mining operations have left acidic and arsenic compounds in
the slag heaps that occasional leach into rivers and despoil them for
long periods of time.