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.

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