This freshwater network is the very lifeblood of all living communities, including human society. For example, the timing and quantity of stream flow and water storage in aquifers and wetlands result in a complex pattern of fast and slow currents, eddies, and backwater pools, creating a rich variety of aquatic habitats that harbor diverse communities of plants and animals. Even periodic natural flooding plays an important role in structuring habitat and providing conditions for various plant and animal species throughout much of a river system. Flooding changes existing vegetation and reconfigures landforms in active river channels, and the silt-laden floodwaters provide the rich soils commonly found in river floodplains . Many plant and animal species that once thrived throughout the Everglades cannot survive in the increasingly saline aquatic environment, and have disappeared from the area.
Problems also arise as a result of the use or rivers and their tributaries as waste streams.
Historically, streams and rivers have served as dumping sites for the refuse, waste water, and runoff from towns and cities across the country. Coastal waters are the ultimate receptacle for these wastes as they flow downstream. Especially vulnerable are estuaries , valuable nursery and feeding grounds for a host of fish, waterfowl, and other wildlife species. Over two-thirds of all commercially important fish and shellfish species, such as flounder, shrimp, oysters, and clams, depend on these shallow, protective coastal waters for part or all of their lives. Migrating birds find essential feeding grounds and resting stops in the salt marshes and mudflats of the Nation's estuaries.