Rivers from source to mouth

A river is a long stream of water that keeps moving.

It always flows downhill. River water is fresh, not salty like the sea. Fish, birds, plants and people all need rivers.

A river is a long, ribbon-like body of fresh water that flows downhill. Gravity pulls it, just as gravity pulls a ball down a slide. Some rivers are wide and deep; others are shallow enough to wade across. A smaller flowing body of water is called a stream, creek or brook.

Not every river flows all year. Some run only in certain seasons, or after a lot of rain. River water is fresh, which means it has less than 1 percent salt, but rivers still carry salts and nutrients that plants and animals need. That is why some of the richest places for wildlife on Earth are found along rivers.

National Geographic describes a river as a ribbon-like body of fresh water flowing downhill under gravity. The U.S. Geological Survey (USGS) puts it in one word: gravity. Water is always trying to move toward the center of the Earth, and in practice that usually means toward the oceans at sea level, so it collects in valleys and low places as rivers and lakes.

Rivers are a key link in the water cycle. Evaporation from the oceans lifts water into the air; rain and snow return it to the land; and much of the runoff gathers in creeks, streams and rivers that carry it back to the sea. The USGS notes that rivers normally empty into the oceans unless they end in a closed lake, which is rare, or are diverted for people's use, which is common. River water counts as fresh, with less than 1 percent salt, yet all the world's rivers together are estimated to carry about 3.6 billion metric tons of dissolved salt to the ocean each year.

Every river starts somewhere. This place is called the source.

Some rivers start from rain or melting snow on mountains. Some start as water bubbling up from the ground. Some start at the edge of a lake. At the start, a river is often small.

The place where a river begins is its source, or headwater. Rain or melting snow high in the mountains can start a river. So can a spring, where water from under the ground bubbles up to the surface. Some rivers begin at the edge of a lake or a large pond.

The mighty Mississippi River starts at Lake Itasca in Minnesota. There it is only about 6 to 9 meters (20 to 30 feet) wide, the narrowest point along its whole length. Near its source the water moves at about 2 kilometers per hour, roughly half as fast as people walk.

A river's starting point is its source, or headwater. National Geographic lists several kinds: rainfall or snowmelt in mountains, groundwater that bubbles up at the surface, or water flowing out at the edge of a lake or large pond.

The Mississippi is a clear example of a lake source. The National Park Service (NPS) says that at Lake Itasca, in northern Minnesota, it is between 20 and 30 feet (about 6 to 9 m) wide, the narrowest point along its whole length, with an average flow of about 6 cubic feet per second. The average surface speed there is about 1.2 miles per hour (about 2 km/h), roughly half a walking pace. Even the length is measured differently: the USGS has published 2,300 miles, the EPA 2,320 and the NPS 2,350 (about 3,780 km), partly because deposition and erosion at the delta keep changing where the river ends.

As a river flows, little streams join it.

Each stream adds more water. So the river gets bigger and bigger. The streams that join are called tributaries.

A tributary is a stream or river that flows into a bigger one. The place where they meet is called a confluence. Most large rivers are made from many tributaries joining together, each one bringing water from its own patch of land.

All the land that drains into a river and its tributaries is called its watershed, or drainage basin. The Missouri River is the largest tributary of the Mississippi, and they meet at St. Louis. The Mississippi's whole watershed covers about 3.2 million square kilometers, about 40 percent of the continental United States, and reaches into two Canadian provinces.

A tributary feeds a larger stream or river, called the mainstem; they meet at a confluence. Geographers name a left-bank or right-bank tributary by looking downstream. The opposite of a tributary is a distributary, a channel that branches away from the mainstem, as happens across many deltas.

The USGS defines a watershed as the area of land that drains all its streams and rainfall to a common outlet. It can be as small as a footprint, and larger watersheds contain many smaller ones; ridges between them are drainage divides. Because a river's size depends on its watershed, big rivers rise and fall more slowly after storms than small ones. The Missouri, itself fed by dozens of tributaries from Alberta and Saskatchewan to the Upper Midwest, joins the Mississippi at St. Louis. The NPS gives the Mississippi watershed as about 3.2 million km² (1.2 million square miles), covering all or parts of 31 states and two Canadian provinces, the fourth largest in the world; the Amazon's drains about 7.1 million km².

On flat land, a river wiggles from side to side. The big bends are called meanders.

Sometimes a bend gets cut off. It becomes a lake shaped like a curve. When the river floods, it leaves mud on the flat land beside it.

Where a river winds across flat land, it makes big bends called meanders. The water flows a little faster on the outside of each bend, so it wears that bank away. On the inside of the bend, the slower water drops sand and mud. Bit by bit, the bend grows wider.

Sometimes a bend is cut off from the river and left behind as a curved lake, called an oxbow lake. The flat land next to a river is its floodplain. When the river overflows, it spreads mud and sand there, which can make the soil good for farming. The huge floodplain of the Amazon River has hundreds of oxbow lakes.

The British Geological Survey (BGS) explains meanders as a balance of erosion and deposition. On a river winding through flat land, water moves a little faster along the outer bank, which gives it more power to wear that bank back and deepen the channel there; slower water on the inner bank drops sediment carried from upstream. So each bend grows sideways and the curve deepens over time.

When a meander is cut off from the mainstem, an oxbow lake remains; National Geographic notes that the Amazon's lowland floodplain is carved with hundreds of them. A floodplain is the flat land from the river's banks out to the valley walls. It grows by aggradation, as floods spread alluvium (silt, sand and gravel), and is reshaped by erosion as the channel shifts. Rivers with a steep gradient and a deep, fast channel can have almost no floodplain. Floodplains and deltas are highly fertile: California's Central Valley floodplain produces about a third of the vegetables and two-thirds of the fruits and nuts eaten in the United States.

The end of a river is called its mouth.

There the river pours into a lake, a bigger river or the sea. The river slows down. It drops the sand and mud it carried. Sometimes this mud makes new land.

A river ends at its mouth, where it empties into a lake, a larger river or the ocean. Here the river slows down. A slow river cannot carry as much, so it drops the gravel, sand, silt and clay it has picked up along the way. Where lots of this piles up, it can build new land called a delta.

Where a river meets the sea, fresh water and salty seawater mix into slightly salty water called brackish water. This area is an estuary. In the Hudson River in New York, brackish water reaches more than 240 kilometers (150 miles) upstream. Many busy port cities, such as Rotterdam in the Netherlands at the mouth of the Rhine, were built at river mouths.

The mouth is where a river enters a lake, a larger river or the ocean. As the current slows, it loses the energy to carry its load, and gravel, sand, silt and clay (alluvium) settle out. Where deposition outpaces removal, a delta forms; National Geographic names the Ganges Delta in Bangladesh and India, about 350 km along the Bay of Bengal, as the world's biggest.

An estuary forms where seawater and river water mix into brackish water; in the Hudson it extends more than 241 km (150 miles) upstream. Mouths are rich places for fish and people alike, and harbor cities such as Rotterdam grew at the mouth of the Rhine. They are also fragile. National Geographic notes that 20th-century river management effectively shut the mouth of the Colorado River: fresh water now reaches its delta only when the reservoirs behind its dams are full, and native species such as cottonwood trees and the Colorado Delta clam are endangered for lack of habitat.

People use rivers in many ways.

Rivers give us water to drink. They water farms. Boats carry things on them. People swim, fish and paddle on rivers too.

People use rivers for drinking water, for watering crops, for making electricity at dams, for moving goods by boat, for fishing and for fun, like swimming and canoeing. All of these uses can change a river's health, so scientists keep checking the water.

On the upper Mississippi, barges carry about 175 million tons of goods each year through 29 locks and dams. More than 50 cities take their daily water from the Mississippi. A drop of water leaving Lake Itasca takes about three months to reach the sea.

The USGS lists the ways people use rivers: drinking water, irrigation, electricity, carrying away wastes (ideally treated), transporting goods and food. Much of the water that homes use returns to rivers through wastewater treatment plants. Each use can affect river ecosystems, which is why scientists such as limnologists monitor them.

The Mississippi shows the scale. The NPS reports that barges move about 175 million tons of freight a year on the upper river through 29 locks and dams, and the EPA says more than 50 cities rely on it for daily water supply. By discharge it is the world's 15th largest river, pouring about 16,792 cubic meters of water per second into the sea; the Amazon, which drains a rainforest, discharges about 209,000 cubic meters per second. Water leaving Lake Itasca takes about three months to reach the Gulf.