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.
Steeper and higher than mere hills, mountains are over 1,900 feet in elevation and typically have steep, sloping sides and rounded ridges and peaks. They tend to be rocky and barren above the tree line, and many mountains get significant amounts of snow in the winter months. They are formed when the Earth’s tectonic plates collide, pushing the crust of the planet higher, forming mountain ranges.