Denver park lake safety: 5 things to know

Here are the top things city employees look for in Denver lakes


Denver parks are home to several lakes throughout the city. Washington Park has several water sources, including two lakes — Grasmere and Smith. The lakes are filled with recycled water. The ditch that runs throughout Wash Park brings new water from a treatment plant into the two lakes.

The Denver Department of Public Health and Environment monitors lakes such as the two in Wash Park by testing them for bacteria, nutrients and more. The department works closely with Parks and Recreation, which is in charge of managing the lakes, said Alan Polonsky, an environmental analyst with the DDPHE.

Lake safety is often a cycle, where one nutrient being high or low can impact other areas of water quality. “Everything effects something else,” Polonsky said.


Nutrients that are in the water are one of the main things that DDPHE tests for, Polonsky said. Nitrogen and phosphorus are the two main nutrients found in lakes.

Because the two lakes in Washington Park are filled with recycled water, the levels of nitrogen have been high in the past. But, more recently, those numbers have been going down. Polonksy said that the Metro Water Reclamation Plant where the lakes get their water has improved its system for removing nutrients, which has helped lower those levels.

The amount of nutrients in a lake impacts the algae growth. Water net algae in particular has been a problem in Grasmere lake for the last several years. The algae tends to latch onto plants that grow in shallow lake bottoms, Polonsky said.

“That's what we saw,” he said. “It's real explosive stuff.”

Algae growth

Explosive algae growth is another thing the DDPHE pays attention to. When lots of algae is growing in a lake, it can spike the oxygen and pH levels of the water.

Polonsky said that analysts with the DDPHE pay close attention to what types of algae are growing in lakes. This year, a viral story about a dog dying in Texas after swimming in water that had blue-green algae in it caused alarm across the country.

Because there was a high level of the water net algae in Grasmere Lake, and there was a good mix of phytoplankton in Smith Lake, Polonsky said it was a small chance that blue-green algae had gotten to dangerous levels in Washington Park.

“It was a pretty good year for the two lakes,” he said.


Although it is not often seen in lakes, the DDPHE does test regularly for bacteria such as E. coli.

Polonksy said that bacteria in lakes or streams can become an issue when wastewater is involved. After a storm, water that has drained could carry bacteria. Because Washington Park gets its water from the ditch, it can act as a buffer between the two water sources, Polonsky said.

Management implications

After DDPHE has tested the water, it gives recommendations to Parks and Recreation on how to manage any issues that arise — high algae growth, nitrogen levels, etc.

Storm water detention areas are one example of managing water before it gets into streams and lakes. Collecting the water in one area allows any unwanted sediment to sink to the bottom.

Both lakes in Washington Park also have solar-powered mixing columns to help keep the water moving.

Water quality

Water quality, Polonsky said, is the “umbrella” category that all the other items fall under. It is the largest factor that impacts the fish and plant life in a lake.

High pH and oxygen levels due to algae growth, he said, “those two can have direct issues with aquatic life.”

Another item that DDPHE tests for, but is not usually a problem in Denver lakes, is metal levels. Most often, copper sulfate can be found in lakes because it is used for algae control. Although those levels of copper are often taken care of by a lake naturally, Polonsky said he prefers a dye method for controlling algae.

The dye turns the water blue, making it harder for sun to penetrate and plants to use it for photosynthesis. It is a nonchemical method for controlling algae growth.


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