Human Activities That Change Natural Processes

A lake that came back

Until the 1920s, Sumas Lake lay in the Fraser Valley east of Vancouver, a large, shallow lake that rose and fell with the seasons and was central to the lives of the Semá:th (Sumas) First Nation. In the 1920s, the lake was drained with dikes, canals and pumps to create farmland. In November 2021, an atmospheric river brought days of heavy rain, a dike was breached, and water poured back into the old lake bed. For a while, the lake was back.

The story shows something important: people change natural processes, and nature often pushes back. Expectation B2.2 asks you to explain how human activities alter physical processes and affect natural events and phenomena in Canada, including in your local region.

1. Cities change the water cycle

When forests and fields are replaced by roofs, roads and parking lots, the land becomes impervious. Rain cannot soak in, so:

• More water runs off, and it runs off faster, through gutters and storm sewers into streams.
• Streams rise higher and sooner after storms, causing flash floods and erosion of stream banks.
• Less water infiltrates, so groundwater is recharged less, and small streams may run low in dry weather.
• Many city streams were buried in pipes or channelized with concrete. In Toronto, for example, Garrison Creek and many other creeks now flow in underground sewers.

Cities also change local climate: dark pavement and buildings absorb heat, creating urban heat islands that can be several degrees warmer than the countryside, especially at night. Severe rainstorms that overwhelm city drains, such as one that flooded roads, basements and parts of the subway system in Toronto in July 2013, show how urban design affects flood risk.

2. Draining and filling wetlands

Wetlands act like sponges: they store flood water, release it slowly, filter pollutants and provide habitat. In southern Ontario, more than 70% of the original wetlands have been drained or filled, mostly for farming and development. Losing them means faster runoff, bigger floods, more erosion and poorer water quality. Sumas Lake is a dramatic example of what can happen when the land's natural storage is removed.

3. Dams and reservoirs

Dams provide hydroelectricity, water supply and flood control, but they change rivers:

• Upstream, a reservoir floods land, forests and sometimes communities and traditional territories.
• Sediment settles in the reservoir, so the river downstream carries less sediment and can erode its bed and banks.
• Flows are controlled, so natural spring floods become smaller. On the Peace River, after the W.A.C. Bennett Dam was built in B.C. in the 1960s, people downstream, including the Mikisew Cree First Nation and Athabasca Chipewyan First Nation, raised concerns that the Peace–Athabasca Delta was flooding less often and drying out.
• Dams block fish migration unless fish ladders or other passages are built.

4. Farming, forestry and mining

• Farming: ploughing exposes soil to wind and water erosion. Tile drainage (pipes under fields) moves water off land faster. Fertilizer and manure can wash into lakes: excess phosphorus, much of it from farmland runoff, feeds large algae blooms in the western basin of Lake Erie (eutrophication).
• Forestry: clear-cutting removes trees that intercept rain and hold soil, which can increase runoff, erosion and landslides on steep slopes. Decades of putting out nearly every forest fire allowed fuel to build up in some forests, which can make later fires more intense.
• Mining and oil sands: removing rock, soil and forest reshapes the land; tailings ponds and waste rock must be managed so they do not pollute water. Companies are required to reclaim land, but restoring a working ecosystem takes a long time.

5. Changing the atmosphere

Burning fossil fuels adds greenhouse gases that are warming the climate, which changes almost every physical process in Canada: permafrost, sea ice, glaciers, wildfire seasons and heavy rain. That is the subject of the next lesson.

Your local region

In southern Ontario, look for: suburbs growing onto farmland and the Oak Ridges Moraine (which stores groundwater), streams in concrete channels, storm ponds in new subdivisions, and green infrastructure such as rain gardens, permeable pavement, green roofs and restored wetlands designed to slow water down again. Your local conservation authority tracks how land use changes affect your watershed.