Water at Work: The Hydrologic Cycle, Drainage Basins and Rivers

One raindrop, three oceans

High in the Rocky Mountains on the Columbia Icefield there is a peak called the Snow Dome. Meltwater from it can flow to three different oceans: west to the Pacific by the Columbia River, north to the Arctic by the Athabasca and Mackenzie rivers, and east to Hudson Bay (and from there the Atlantic) by the Saskatchewan River.

Where water goes, and what it does on the way, is one of the most powerful forces shaping Canada. In this lesson you will follow water through the hydrologic cycle, through Canada's drainage basins, and through the landforms it carves and builds.

The hydrologic cycle

Water is always moving:

1. Evaporation: the sun heats water in oceans, lakes and rivers, and it rises as invisible water vapour. Transpiration adds more vapour from plants.
2. Condensation: as the air rises and cools, vapour condenses into tiny droplets, forming clouds.
3. Precipitation: rain, snow, sleet or hail falls to the ground. In much of Canada, a big share of yearly precipitation is stored for months as snow, then released in a spring melt.
4. Runoff and infiltration: water flows over the land into streams (runoff) or soaks into the ground (infiltration) to become groundwater, which slowly feeds wells, springs, lakes and rivers.
5. Storage: water can be stored for days (in the air), years (in lakes), or thousands of years (in glaciers and deep groundwater).

These are hydrological processes. They interact with climate (how much rain and snow, how fast evaporation happens) and with landforms (steep slopes create fast runoff; sandy moraines soak up water).

Drainage basins: where does the water go?

A drainage basin (or watershed) is all the land that drains into one river system. A drainage divide, or height of land, separates one basin from the next. Rain falling on one side of a divide ends up in a completely different place from rain falling a few metres away on the other side.

Canada's rivers drain into five ocean drainage areas:

• Atlantic Ocean: the Great Lakes and St. Lawrence River, plus rivers of the Maritimes and Labrador. Southern Ontario and southern Quebec drain here.
• Hudson Bay: a huge area including the Saskatchewan, Red, Nelson and Churchill rivers in the Prairies and northern Ontario's rivers that flow north into Hudson and James bays.
• Arctic Ocean: the Mackenzie River system, Canada's longest, flowing north through the Northwest Territories.
• Pacific Ocean: the Fraser, Columbia and Yukon rivers and other rivers west of the continental divide in the Rockies.
• Gulf of Mexico: a small area in southern Alberta and Saskatchewan drains south through the Milk River to the Missouri and Mississippi.

Spatial significance: much of Canada's river water flows north into Hudson Bay and the Arctic Ocean, far from the southern cities where most people live. Having lots of fresh water is not the same as having it where people need it.

Rivers carve and build the land

Running water does three jobs: it erodes (wears away), transports (carries) and deposits (drops) material.

• Fast water (steep slopes, floods) erodes and carries more. It cuts V-shaped valleys and canyons, such as the Fraser Canyon in British Columbia.
• Waterfalls form where a river flows over hard rock on top of softer rock. At Niagara Falls, the water undercuts soft shale beneath a hard dolostone cap; the cap breaks off and the falls move upstream. Over thousands of years this has carved the Niagara Gorge, about 11 km long.
• Slower water drops sediment. On flat land, rivers swing into loops called meanders. The water erodes the outside of each bend and deposits on the inside. Eventually, a loop can be cut off to form an oxbow lake. In floods, rivers spread silt over floodplains, making rich soil, as along the Red River.
• Where a river meets a lake or ocean and slows down, it builds a delta, such as the Fraser River delta at Vancouver and the huge Mackenzie Delta on the Arctic coast.

Waves, tides and coasts

Oceans and large lakes shape coastlines too. Waves erode headlands, cut sea caves, and leave arches and sea stacks standing alone. At the Bay of Fundy, between New Brunswick and Nova Scotia, the highest tides in the world (up to about 16 m) rise and fall twice a day. The moving water erodes soft sandstone cliffs into the famous flowerpot-shaped stacks at Hopewell Rocks. Waves also move sand along shores, building beaches, spits and sandbars, such as Point Pelee in Lake Erie.