Rivers are flowing water habitats, which can vary from an icy mountain stream to a huge tropical river. Lakes are still-water habitats, ranging from tiny rain pools to giant lakes many thousands of feet (meters) deep. Each of these habitats has its own types of plants and animals, depending on factors such as how fast the water moves and is replaced, the nature of the rocks and soil, and the chemical makeup of the water itself.
Rivers and lakes are usually freshwater habitats that support many forms of life, from algae and plankton to insects and fish to larger animals. The food chain and the interdependence of the plants and animals in these habitats can be disrupted by pollution, pesticides, and erosion.