Fighting the summer slime at Henderson Lake

As we move into hotter days, the City of Lethbridge is working to keep Henderson Lake as pristine as possible.

Algae is a common summer concern at the lake – and extreme heat makes it worse. It mostly comes down to phosphorous. Here’s the quick science behind it: when the lake warms up, the surface water heats while the deeper water stays cold, and the two layers stop mixing. The cold bottom layer eventually runs out of oxygen. Once that happens, the sediments let go of the phosphorous they normally hold onto, and it slips back into the water.

Too much phosphorus allows algae to grow faster than the lake can handle. That quick growth creates algae and cyanobacteria blooms – more commonly known as blue-green algae. That makes the water dangerous for people and animals. When the bloom dies off, it also sucks oxygen out of the water, stressing or killing fish.

The City of Lethbridge takes several proactive measures to try to avoid dangerous lake algae:

• Students and staff from Lethbridge Polytechnic sample and monitor lake water every two weeks.
• There are 15 aerators that help keep oxygen in the water column. This helps with water circulation, which supports fish survival.
• Five SolarBees help mix and move the water. These are solar-powered, floating water circulators. They continuously draw water from specific depths and gently distribute it across the surface. This helps prevent stagnation and control blue-green algae.
• The water is treated with Reward, an herbicide that targets excess plant growth that can also result in toxic algae.

More aggressive algae treatments, like copper or pesticides, aren’t an option because the City doesn’t have a provincial permit to use them. The City also doesn’t dredge the bottom of the lake. It’s very expensive and requires extensive permitting that we don’t have.

There is help coming, though! We’re awaiting federal approval for an imported product that targets excess phosphorous. The application was submitted last winter, and we hope to have a decision by mid-June. The unfortunate news is, we won’t be able to use the treatment until next spring.

This means as we head into summer 2026, we may see more instances of algae and cyanobacteria blooms in Henderson Lake. The City of Lethbridge will notify the public if any significant blooms occur, including safety steps to keep you and your pets safe.

For Public Inquiries:
Call or chat 311 | Monday to Friday, 8 a.m. to 4:30 p.m.