The Ottawa River is at its lowest level in decades in the National Capital Region. This fisherman was trying his luck when the water was higher, in May. (Pawel Dwulit/Canadian Press)

The Ottawa River in the National Capital Region is at its lowest level in decades.

"Water levels are much below normal for this time of the year and will remain constant or decrease slightly this week," the weekly report by the Ottawa River Regulation Planning Board said on July 13.

The water level is down nearly 40 centimetres at Britannia Bay, compared with the long term July average, and down 22 centimetres at Hull, board statistics show.

A dry winter followed by a hot summer has weakened the flow, said Ottawa Riverkeeper Meredith Brown. And that could be bad news for fish.

The Ottawa River is one of the warmest in North America because the water gets heated up by the sun in a series of shallow lakes behind more than a dozen hydro dams. As the water level drops, the water gets even warmer, and the sun's rays cause plants and algae to flourish.

That reduces the oxygen in the water, a huge factor in fish kills, said Brown.

But upstream from Ottawa, the river is not down as much. Joe Kowalski, who pioneered whitewater tourism on the river, said that at Beachburg it's still about 17 centimetres above the lowest measurement in the past 36 years, with little impact on rafting.

Hydro companies aren't worried either. Ontario Power Generation and Hydro-Québec said there's still enough water passing through their dams to provide electricity.