LETHBRIDGE – For many dryland farmers, irrigation brought an oasis to the desert.
But a University of Lethbridge hydrologist said irrigation also has negative affects because it can dramatically alter rivers and their ecosystems.
Stewart Rood spoke about the effects of irrigation on rivers at the Alberta Irrigation Project Association annual meeting in Lethbridge Nov. 26.
Southern Alberta’s 13 irrigation districts depend on rivers flowing from the Rocky Mountains to supply them with water for 1.3 million acres of land. Everything from forage crops to vegetables are cultivated.
Read Also
The Western Producer Livestock Report – August 28, 2025
Western Producer Livestock Report for August 28, 2025. See U.S. & Canadian hog prices, Canadian bison & lamb market data and sales insights.
Extra water captured during peak flow periods in the late spring is stored in reservoirs to create the only water bodies found in the southern half of the province. There are no natural lakes in the southeast.
Bringing water to farmland and rural communities from Calgary to the American border has brought economic and environmental benefits for the south. It provides drinking water, irrigates crops, creates recreational areas around reservoirs and, when tended properly, allows wildlife to thrive in habitats along rivers, canals and reservoirs.
But the environment has been hurt in some cases, Rood said.
Reservoirs holding irrigation water were created by flooding. That has led to the loss of some of the most valuable trout fisheries and key wilderness areas in Canada.
Altering the course of a river upstream with flooding affects the areas downstream as well.
Water temperature and speed may alter, making it unsuitable for aquatic life. During hot, dry periods of high water demand, some rivers are reduced to a trickle. This is partly because upstream waters are diverted and the supply is overallocated at other points. The result is loss of habitat, trees, recreational opportunities and stream health.
During low flow periods there is an indirect problem with bacteria growing in rivers. Some of that bacteria is responsible for gastrointestinal upsets in people and animals.
Building dams like the one on the Oldman River is supported by many who want to regulate stream flows during hot, dry periods and reduce flood danger, said Rood.
Floods which struck southern Alberta in June 1995 happen naturally about twice a century. The force of that flood was lessened by about 20 percent which was probably not enough, said Rood.
“It’s quite clear that the reservoirs in southern Alberta have not and are not able to dramatically reduce flooding.”
Similar flooding occurred in that area in 1902 and 1908.
As time goes by people are learning the mechanisms of waterways.
One of the first steps taken recently by Alberta Environment is to establish minimum flow needs, which means the river should only be allowed to drop to its minimum occasionally.
For rivers like the Oldman, a minimum instream flow requirement has been established, which states the smallest amount of water required to keep the ecosystem and economic elements going.
“Originally people would ask ‘how low can we go’?” said Rood.
That kind of thinking was wrong and now people realize there should be a target flow that fits with the river’s natural variation to keep it healthy.
