![]() ![]() Rivers and lakes make up only a small fraction of Earth’s freshwater resources. Earth’s freshwater sources are mostly locked within glaciers and ice caps and as groundwater. Figure 9.2: Most of the Earth’s water is found in oceans and is therefore saltwater. Additional water resources are also found in the ground, and will be discussed in another chapter ( Figure 9.2). The majority of water is found in oceans, but freshwater can be found in lakes, rivers and trapped away in glaciers and ice sheets. At Earth’s surface, this cycle, powered by the sun, operates easily since water can change form from liquid to gas (or water vapor) quickly. This movement of water through the Earth System is referred to as the hydrologic (water) cycle ( Figure 9.1). Water is continually recycled through the atmosphere, to the land, and back to the oceans. ![]() What happens to the rain in the grassy area? Much of it will infiltrate, or soak into the ground. ![]() Where does the water from the parking lot go? Much of it will flow across the surface and eventually join a stream. What happens to water during a rainstorm? Imagine that you are outside in a parking lot with grassy areas nearby. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years. Figure 9.1: Earth's water is always in movement, and the natural water cycle (hydrologic cycle), describes the continuous movement of water on, above, and below the surface of the Earth. Some of these processes are slow and result in landscapes worn down over time others, such as floods, can be dramatically fast and dangerous. Water can sculpt the landscape dramatically over time, by both carving canyons and depositing thick layers of sediment. How many times a day do you take water for granted? Do you assume the tap will be flowing when you turn on your faucet? That the shower will turn on, the toilet will flush, and you’ll have water to cook your meals? Not only is water necessary for many of life’s functions, it is also a considerable geologic agent. It can also be called a horseshoe lake, a loop lake, or a cutoff lake.\) An oxbow lake gets its name from the U-shaped collar placed around an ox's neck to which a plow is attached. These lakes eventually dried up to create acres of land for farming, housing, and industry. The Mississippi River is shorter now than it was in the 19th century, for instance, because engineers have cut off hundreds of meanders. Giant river otters ( Pteronura brasiliensis) frequently live in the Amazons oxbow lakes, and feed on about five kilograms (11 pounds) of fish per day. ![]() Along the Amazon River in South America, oxbow lakes are common and their still waters provide a unique habitat for plants and wildlife. Oxbow lakes can be rich wildlife habitats. Oxbow lakes often become swamps or bogs, and they often dry up as their water evaporates. There is no stream or spring feeding the lake, and it doesn't have a natural outlet. This means that water does not flow into or out of them. Oxbow lakes are the remains of the bend in the river. Erosion and deposition eventually cause a new channel to be cut through the small piece of land at the narrow end of the meander. Silt and sediment build up on convex banks. The banks opposite the concave banks are called convex banks. The force of the rivers flowing water wears away the land on the meanders concave banks. The corners of the curves closest to each other are called concave banks. Meanders that form oxbow lakes have two sets of curves: one curving away from the straight path of the river and one curving back. On these plains, rivers often have wide meanders. Oxbow lakes usually form in flat, low-lying plains close to where the river empties into another body of water. The meander becomes an oxbow lake along the side of the river. A lake forms as the river finds a different, shorter, course. An oxbow lake starts out as a curve, or meander, in a river. ![]()
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