ivers are always picking up and dropping off materials. Where the river moves fast, earth is excavated away — the more powerful the current, the larger the transported material. When the current slows, material settles onto the riverbottom. Healthy rivers are always carving new channels and excavating new banks. Healthy rivers wander, regularly making new channels and overflowing their banks. The flat land beside a river, called the floodplain, reveals where water once flowed: Floodplains are flat because the river carved away the land and deposited new sediments over time. Buildings and roads in floodplains are regularly threatened by floods and erosion from rivers that are just behaving naturally.

Beavers are ecosystem engineers. Their dams slow down and oxygenate the passing river water, improving the health of the whole aquatic system. The dams also provide new habitat for fish and invertebrates. Even after the dams break and wash away, rocks piled up against the dam often remain, creating new riffles and features that improve habitat diversity. Beaver dams strain out suspended sediment and some pollutants, making them a natural filter that can improve water quality. They also store water behind them to help reduce flooding downstream.

If storms are natural, stormwater must be too, right? Not necessarily. Rainfall in a natural setting is absorbed into the soil like water into a sponge. But when rain falls on pavement, rooftops, or compacted ground, it instead rushes quickly to a nearby stream or storm drain. The larger the impervious area, the more stormwater is created. Powerful surges cause erosion, flooding, and infrastructure damage, while picking up all sorts of stuff along the way like fertilizers, road salt, pesticides, dog poop, and vehicle fluids. Major stormwater events can overwhelm city sewers, causing sewage to discharge right into the river! As a result, water quality at river mouths or confluences can be much worse than upstream.

Many invertebrates rely on small spaces and cracks between rocks in the stream bottom in order to hide from predators, lay eggs, and forage for food. Rocky headwaters offer plenty of this habitat. But farther downstream (like here at the confluence), sediment settling out of the water column can plug up these spaces, destroying this important habitat and the creatures who need it. Sedimentation is a natural process that always occurs to some degree in slow, downstream sections of a watershed. Some aquatic organisms even prefer sandy substrates. But development, agriculture, and stormwater-related erosion can cause massive, unnatural pulses of sediment that quickly bury rich and diverse habitats under sand and silt.

Riffles and waterfalls infuse streams with oxygen — and cold water can hold twice as much oxygen as warm water. The shaded, turbulent waters of healthy headwaters streams are fed by groundwater emanating from cracks in the cool underground bedrock. This cold, shaded, oxygen-rich environment is a fantastic place for aquatic invertebrates, and it supplies the whole downstream watershed with a clean, healthy water source. When headwaters lose their shade due to forest clearing or other development, the resulting rise in water temperature can make the stream uninhabitable to many fish and insects.

The trees over a healthy headwaters stream provide a buffet for aquatic life. Insects falling from the treetops become food for fish. Leaves fall and decompose, becoming a nitrogen source for the aquatic ecosystem. As fallen leaves stack up against river rocks like pages of a book, they provide shelter for many organisms. And many aquatic insects need to go ashore and climb a tree to finish their life cycle as adults.

Some rivers hold more diversity underwater than forests hold on land. Every aquatic species prefers different conditions. Some species need slow water, and some need fast water. Some need large cracks between rocks, while some need sandy bottoms. Many fish require submerged boulders and tree trunks in which to hunt or hide. Some creatures depend on colder water, and others slightly warmer. Some prefer rough water, and others prefer deeper, slow-moving pools. A healthy river system can contain all this habitat diversity in just a short stretch.