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Introduction to Rivers & Streams

White water rafting on the Fraser River in British Columbia

White water rafting on the Fraser River in British Columbia (powerofforever, iStockphoto)

White water rafting on the Fraser River in British Columbia

White water rafting on the Fraser River in British Columbia (powerofforever, iStockphoto)

Let's Talk Science

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Learn about the physical geography, chemistry and ecology of rivers and streams.

The Earth is made up of many different ecosystems. The ecosystems on Earth can be divided into two groups. Ecosystems on land are called terrestrial. Ecosystems in water are called aquatic. The two main kinds of aquatic ecosystems are freshwater ecosystems and marine ecosystems.

Freshwater ecosystems can further be divided into lentic ecosystems and lotic ecosystems. This Backgrounder is about lotic ecosystems.

Lotic Ecosystems

Lotic ecosystems are in flowing bodies of freshwater. The word Lotic comes from the Latin word lotus which means ‘washed’. Lotic ecosystems can be as small as tiny springs and as large as rivers several kilometres wide. 

Whatever their size, lotic bodies of water have something in common. They all flow in one direction. Water always flows from the headwaters, which are the source of the river or stream, to the downstream terminus, where the river or stream ends.

Did you know?

Scientifically speaking, all running water is a 'stream'. Even the mighty Mackenzie River is just a big stream. But there's a popular saying:  "you can step over a brook, jump over a creek, wade across a stream, and swim across a river."

Physical Geography

Streams are natural watercourses. They flow over the surface of land in channels and drain areas of land. The location, size and speed of a river is determined by several things. These include the availability of water, the size of the channel and the slope of the land. This is why the term ‘river’ includes all kinds of watercourses. These range from the tiniest creeks to Canada’s Mackenzie River. The Mackenzie River is the 10th longest river in the world!

Image of a creek and a river
Left: Sprout Creek. Right: Floatplane on the Mackenzie River (Sources: Public domain image by Juliancolton via Wikimedia Commons and Public domain image by David Adamec via Wikimedia Commons).


How a river shapes the landscape around it depends on the volume of water and the speed of its flow. 

Fast-flowing water can erode rock, soil and organic matter. In some cases, a river can carve deep valleys into the rock around it. This often happens near a river’s headwaters. 

Slow-moving water can deposit materials like silt and dead plants into a river. This usually happens downstream, near the mouth of the river. The mouth of a river is where it joins up with a lake or an ocean. Deposits at the mouth of a river may create a landform called a delta.

Horton River delta
Horton River Delta in the Northwest Territories (Let’s Talk Science using an image by NASA Earth Observatory).

Rain, melted snow and groundwater all affect how much water flows in a river. The flow of a river can change a lot from season to season and year to year. In Canada, river levels are highest in the spring. This is when flooding happens more often. This is because snow melts but the soil is still frozen. It can’t absorb the extra water. Heavy rains can also cause high water levels and flooding in smaller rivers and streams. 

In Canada, low river levels and flows often happen in late summer. This is when precipitation is low. Summer is also when water evaporates more easily and plants use more water. River levels are often low in winter as well. This is because they are covered in ice and precipitation is in the form of snow, not rain.

Drainage Patterns

The area of land that supplies water to a river is called a watershed or a drainage basin. This water may come from rain, melted snow and ice, or groundwater. One river's watershed is separated from the watersheds of neighbouring rivers by higher lands called drainage divides or watershed divides.

Saskatchewan River drainage area map
The drainage basin of the Saskatchewan River (Source: Karl Musser [CC-BY-SA] via Wikimedia Commons).

Small drainage basins supply water to streams. The drainage basins of several streams often combine to make up the drainage basin of a river. The drainage basins of several rivers combine to make up regional watersheds. These in turn join other regional watersheds to form continental watersheds. These are also known as ocean watersheds.

Rivers in Canada are part of five continental watersheds. The rivers in each continental watershed flow into the ocean, or another body of saltwater. Rivers in Canada flow into the Pacific Ocean, the Arctic Ocean, the Atlantic Ocean, Hudson Bay or Gulf of Mexico.

Drainage map of Canada
Drainage regions of Canada (Source: Statistics Canada).

Measuring River Flow

In the spring you might hear an announcement like "Bear Creek is expected to crest later today at 6.3 meters." The 6.3 metres the announcer is referring to is called the stream stage

Stream stage is the height of the water surface above an established mark which is considered to be zero. The zero level is often close to the stream bottom. The stream stage can be read off a tool called a stream gauge. It can also be recorded electronically by sensors placed in the stream. These send information about the stream stage to a data centre.

Stream stage is important because it can be used to calculate stream flow or discharge. Stream discharge is the volume of water flowing in the stream at any moment. 

Stream gauge in a small creek
Stream gauge (©2002 Derrick Beach, Fisheries and Oceans Canada. Used with permission.).

Discharge and water levels are important. Scientists can use them to manage water resources properly. Here are some of the ways scientists use this information:

  • To reduce damage from floods. Scientists can map floodplains. Floodplains are areas near rivers which often flood. They can also create canals to flood water.
  • To design and build structures near rivers. These include bridges, roads and culverts. A culvert is a tunnel that allows water to pass under a road.
  • To plan and run environmental programs. This could be related to water quality, fisheries and wildlife habitats.
  • To make sure that water resources in Canada are developed in a way that conserves and protects the environment.


Even the healthiest rivers have some dissolved minerals. All water has minerals like sodium, chloride, calcium, magnesium and potassium. But how do they get into rivers? 

Dust, volcanic gases and natural gases like carbon dioxide, oxygen and nitrogen can combine with water in rain. When toxic materials like sulphur dioxide and lead are in the air, they also become part of rain. When rain reaches the Earth's surface, it flows over and through the soil and rocks. This is called surface runoff.

Scientist in river taking water samples
Scientist measuring water quality (Source: damircudic via iStockphoto).


As surface runoff flows over land, it can dissolve and pick up materials. If the soil has a lot of limestone, the surface runoff will have a lot of calcium carbonate. This is because limestone is made of calcium carbonate. And calcium carbonate can dissolve in water. 

In the Canadian Shield, there are large areas without much soil. These areas don’t have many minerals that dissolve in water either. Because of this, the rivers and lakes in these areas have very low amounts of minerals. 


Many plants and animals live in or near rivers. Some species are only found in lotic habitats. Some insect species have developed adaptations so they can fight against high currents. 

This is the case for blackflies. Black fly larvae attach themselves to rocks on the river bed. Their mouthparts form a fan shape. They put this into the current to catch the microscopic plants and animals to eat. 

Black flies attacking a canoeist on the Dubawnt River, Nunavut
Black flies attacking a canoeist on the Dubawnt River, Nunavut (Source: NicolasPerrault [public domain] via Wikimedia Commons).

You may have seen salmon swim up rivers. This happens during their spawning season, when they need to lay their eggs. Salmon live most of their lives in the sea. But they always return to the river where they were born. So it’s important to conserve rivers to make sure salmon survive. Sometimes, when a river is blocked by a dam, fish ladders are built to allow salmon to pass over it.

Salmon Run at Bowmanville Fish Ladder (2020) by Thomas Yee (4:32 min.).

Did you know?

An eel’s life cycle is opposite to a salmon’s. An eel lives most of its life in freshwater but often goes to the ocean to breed.

Scientists use many different kinds of organisms to measure the quality of water. These include invertebrates, algae, zooplankton, fish and aquatic macrophytes. Aquatic macrophytes are large plants that grow in or near water. These include cattails, bulrushes and pondweeds. 

Scientists often use benthic invertebrates to measure water quality. This is a group of bottom-dwelling aquatic animals that don’t have backbones. Scientists use these organisms to measure water quality because they are sensitive to many different substances.

scientist in river sampling benthic invertebrates
Sampling benthic invertebrates (Image ©2002 Derrick Beach, Fisheries and Oceans Canada. Used with permission.).



Learn More

What is Acid Rain?
This article by Let’s Talk Science explains acid precipitation and some of its effects.

Minerals and Rocks
This backgrounder from Let’s Talk Science will teach you the types and characteristics of rocks and minerals, and the rock cycle.

Protect your watershed - An interactive guide to taking action 
This interactive map by Canadian Geographic helps you learn about your local watersheds.



Government of Canada. (n. d.) Extent of Canada's wetlands

Government of Canada. (n. d.) Water and the environment.

Government of Canada. (n. d.) Water quality in Canadian rivers.