In most parts of the U.S., Canada and Australia, a "creek" refers to a small or mid-sized stream, often draining into a river. Creeks derive from the confluence of smaller brooks born of springs or surface runoff.
Creeks descending through highlands are typically fast-moving, clear, and, because of aerating rapids, rich in dissolved oxygen. As their gradient gentles, streams tend to slow down, lose some oxygenatio and become murkier with sediment.
Aquatic creatures partition themselves in creeks partly based on current dynamics. Friction slows down water along the bottom and the banks, while in the center the flow hastens. The calmer waters harbor greater sediment buildup, which attracts burrowing invertebrates.
In many fast-moving creeks, a substantial portion of energy comes from leaves falling on the surface. These are broken down by invertebrate decomposers, which in turn are fed upon by fish, crustaceans and other creatures. Large fish, wading and diving birds, and semi-aquatic mammals like mink and river otters are examples of top-level predators in many creek ecosystems.
Diadromous fish--those cycling between freshwater and saltwater--form an important component of some creek ecosystems. Some, like salmon and lampreys, live in the ocean and spawn in interior creeks. Others, like American eels, live in freshwater and spawn in the ocean.
