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Rivers Facts

mountain riverThe River

A river has a life just the same as any living thing. A river is born at the headwaters and finishes up at its mouth. Come along for the trip as we visit the entire river. Later we will learn about unique aspects of rivers, the cycle of water on the earth, and the rivers of Idaho.

melting iceHeadwaters

It starts with a drip. Probably from melting snow high in the mountains during the warming of spring. The drips collect together until they form a small puddle. This puddle begins to run down the slope of a mountain in a very small trickle. This is the start of a river. icicleAlthough very small, this is where it all begins. This is called the headwaters or the source. It doesn’t start in one specific place, but in many locations generally around the highest part of a mountain.

Creek, brook, brooklet, crick, stream, rivulet…

winter meltwinter flowIt is known by many names, but it means the same thing. As the water begins to form the basis of a river, it starts out very small. Water from several locations begins to collect into miniature rivers. They may be only an inch across or they could gather into a larger version that may be several feet across. The water is controlled by gravity to move down the mountain. And all along its path it finds more water also on the same journey. One stream will find more and join together on the way to the lowest point in the trip.


tributaryThe dripping becomes a trickle, a trickle becomes a stream and streams collect together to make a river. But it doesn’t stop there. Rivers will find each other too and assemble into a wider and bigger river. Along the way, rivers will gather together just the same as the streams gathered into one. When rivers form this type of a family, we call each of the small rivers a tributary. Where they meet is a fork. At the fork, a small river flows into a larger river. One river can take all of the rainwater from a given area and move that water off to drain away. This is known as the drainage basin. The picture shows an overhead view of tributaries flowing into a larger river, much the same way that branches of a tree all connect to the trunk.

dripFascinating Facts

  • There are more than 250,000 rivers in the United States.
  • The Mississippi is the largest river in the U.S.
  • Water covers three fourths of the earth’s surface.
  • Most of that water is permanently frozen or salty.
  • Antarctica holds over 90% of the word’s fresh water.
  • The amount of water on the earth has remained the same for two billion years.


river mouth

A river or tributary must empty into a larger body of water somewhere. It might be another river, a lake, or an ocean. The place where river water empties into another body of water is known as the mouth. The mouth of a river can be a location for deposits of soil, combining different conditions of water and for wildlife to gather. Sometimes the river widens at the mouth and spreads out at this point. Sand and other materials can build up at the mouth.



Most of the rivers in Idaho head for the Columbia River. But the Columbia empties into the Pacific Ocean. It is the joining of the Columbia with ocean water that we call the mouth. Here, fresh water mixes with salty ocean water. This place is called an estuary. The water in an estuary is often not fully salt water, but would not be good to drink because enough salt has invaded it to give it an odd flavor. This water is known as brackish water.

river cutting through sedimentary layersParts of a River

A river never moves from its headwaters directly to the mouth without changing the land that it travels through. The changes that a river makes upon the land help to define a river’s shape, size and even its beauty. Let’s take a look at some of the parts of a river and how it becomes this way.


morainglacierWay up in the mountains of some parts of the world sits the glacier. Years of snow and ice have accumulated over time and have built up a thick, heavy mass of frozen material. Some of this never really melts or it might partially melt and then refreeze. A glacier is actually a river of snow and ice which moves very, very slowly down its coarse. As it moves, it can drag soil and rock along with it. A glacier is great at carving a valley as it travels along. Sometimes a glacier can create a basin or bowl shaped formation. A glacier can move rocks and deposit them miles from their original source or location. Rocks, gravel, and sand from glaciers have been found sitting lonely and far away from the place of a prehistoric glacier. This discarded material is called a moraine and is used to identify glaciers of the past.


eroded hillside

siltMoving water is a powerful force and can wear away soil and rocks. Soil washes down steep slopes especially when there are no plants or trees to hold the soil in place. The moving of soil and rock is called erosion. Erosion is responsible for filling rivers with mud after heavy rain or after a forest fire. This can choke out fish and make the water undrinkable for other wildlife. The moving soil in the river will also act to erode additional rocks and soil. The soil and water can bounce sharp edged rocks and pound them with sand and gravel. This continues the erosion process. When the rocks have worn down to smooth edges, they are easily identified as a characteristic known as river rock.

river rocksGood soil that plants need for holding them in place and feeding them can get removed far away from the plant. Those plants will die. Erosion can also be caused by wind, plant life, animals and humans. Each of these can stir up soil and cause it to move to another location.


valleyErosion is also responsible for creating valleys in mountains. The V shaped grooves are created by water eroding soil from a hill or mountain in a short period of time. This swift means of taking soil away from the mountain often defines the shape of a peak and creates the highs and lows of a mountainside. The U shape of an older valley is evidence of erosion that has taken place over a great period of time where additional erosion from rocks, sand and gravel has moved much more material from the valley floor.


Waterfalls are often some of our favorite scenery in nature. waterfallWe take pictures of them and build parks for people to enjoy them. Some people even create their own waterfalls in their backyard. But nature’s waterfalls are just another sign of the power of moving water. Waterfalls, because of their speed, can move huge amounts of rock and soil. A waterfall can dig a hole at the bottom of its flow known as a plunge pool. The soil and rocks that once sat at the bottom of the waterfall have been moved on down stream. If you could stop the flow of the water you could see that there is an indentation right below where the water drops. The moving water usually prevents us from viewing this pool. The moving water can also wear away the rocks at the top of the waterfall and the shape of the waterfall can change as the years go by.

precipitationMore Fascinating Facts

If all of the world’s water could fit into a gallon sized container, only 1 tablespoon of that water would be drinkable, fresh water. 1.2 billion people in the world do not have access to clean water.


As a river flows over years and years, it can create a new path for itself. It can wind its way around rocks and trees and then change its route another year. It can pick up soil material from one area and drop it in another area, creating a new route for it to follow. This twisting and turning of a river is known as a meander. If conditions are just right, a river can abandon one U shaped section of its meander to create a lake. This is known as an oxbow lake. See the oxbow in the picture? It is in the far right side of the meander.

Flood Plain

flood plainDuring the spring, melting can create more water than a river can carry. Many years ago this would cause the banks of the river to overflow and the water would spill out over the nearby land causing a flood. In some areas this could repeat itself every season. By watching where the water would end up, farmers could predict the areas that would be affected and plan where to build houses, where to plant crops and even where to keep their cattle. This area of flooding is known as a flood plain. Today we can control the water better through the use of dams. Water can be stored behind the dam and let down as needed. This doesn’t stop flooding all of the time, but it can be helpful.


river canyonAs a river travels and carries away rocks and soil, it can create a deep groove in the earth’s surface. All along the sides of the river, the groove can get deeper and deeper. In time, this can create a canyon. Depending upon the type of rock along the sides of the river, a canyon can have sharp cliff-like sides. Some canyons are very famous, such as the Grand Canyon or the Snake River Canyon.

river deltaDelta

At the mouth of many rivers a delta can form. A delta is made of the soil and debris that the river has washed down its entire route. When the river comes to the mouth, the speed of the water often slows and it allows the material to pile up there. Deltas can be all shapes, but are usually formed in a fan shape or a triangle. Over time, this material can become very thick. Even thick enough to build on. The city of New Orleans, Louisiana is built on the delta of the Mississippi River where the river meets the Gulf of Mexico.

water cycle

The Water Cycle

Rivers are just a part of a bigger system of the earth known as the water cycle. All of the water that has ever existed or ever will exist is here on the earth today. The same water that your favorite dinosaur drank might be the water you drink later today. Some of the earth’s water is frozen, some of it is in a gaseous form and then some is liquid. Water can be in any of these forms here upon the earth. Some water is even part of living things. It is in your body, it is in the trees, it is in your food and even in the air. This water goes through a constant cycle of movement which cleans, redistributes and stores it for later use.

It works like this…the water cycle moves in three basic parts: evaporation, condensation, and precipitation.

Let’s start with evaporation…water in lakes or in the oceans is heated by the sun. This heated water evaporates into the atmosphere in the form of water vapor. When the water vapor reaches a certain height in the atmosphere it begins to cool down. This causes it to change back into water droplets. This is called condensation or we call it clouds. We think that they look like white, fluffy marshmallows, but they are really just a collection of tiny water drops. If the conditions are just right and enough water droplets have collected, then they get too heavy for the atmosphere to hold and they begin to fall to the ground. If the air is cold enough, they might fall as snow; warm enough and they will fall as rain. Sleet and hail are other ways that water can fall to the earth. All of these are called precipitation. Much of the precipitation falls over mountainous areas and collects as snowcaps or glaciers. When these begin to melt in the spring and runoff, we have rivers!! The rivers flow to a lake or an ocean and the process starts all over again.

waterStill More Fascinating Facts

  • A corn field gives off 4,000 gallons of water per day per acre.
  • One gallon of water weighs a little over 8 pounds.
  • There are nearly 100,000 glaciers in Alaska.
  • Ice worms live on glaciers and eat pollen, insects, mineral and bacteria that is deposited there by the wind.

river island


Let’s take a look at some of Idaho’s rivers.



Drainage Basin



1040 miles
(1670 km)

109,000 sq miles
(282,000 sq km)

Southern Idaho - largest


90 miles
(145 km)

9645 sq miles
(24,980 sq km)

Central Idaho


95 miles
(150 km)

2122 sq miles
(5496) sq km

Western Idaho


420 miles
(680 km)

1206 sq mi
(3124) sq km

Northern Idaho


62 miles
(100 km)

3240 sq miles
(8390) sq km

Western Idaho

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