Streams are the small bodies of running water that link the terrestrial landscape to larger aquatic ecosystems. They are organized into drainage networks, and hydrologic connectivity of the drainage network is important to maintaining the biological integrity of a stream. Streams serve as distinctive habitat for specialized aquatic invertebrates, and also provide refugia and spawning grounds for many species. Tight physical connections to the terrestrial landscape mean that stream chemistry integrates all the biogeochemical processes occurring in a watershed. Export of carbon and nutrients from watersheds via streams provides an important contribution to the nutrient loading of downstream aquatic ecosystems. Biogeochemical processes occurring in the stream and riparian zone can result in removal of organic carbon and nitrate from surface waters, thereby providing an important ecosystem service that helps prevent eutrophication of downstream waters. Because they integrate the effects of a wide variety of processes occurring in their watersheds, the physical, chemical, and biological characteristics of streams can serve as important indicators of overall ecosystem health.
