Sloan’s Lake Loop (Photo) Credit Amy Aletheia Cahill (flickr)

11. Sloan’s Lake Loop

Located in Denver’s city limits is Sloan’s Lake Loop, featuring a lake. The urban trail system circles Sloan’s Lake and is great for many trail uses, including walking, running, bird watching, and biking.

Distance: 2.6 miles

Difficulty Rating: easy

When to go: year-round.

Photo Credit: Amy Aletheia Cahill (flickr).

Denver health officials closed Sloan's Lake indefinitely Thursday after finding increased spread of toxic algae-bloom.

Blue-green algae is caused by a bacteria and can poison people, pets, livestock, wildlife, birds and fish. Its presence in Sloan's Lake west of downtown prompted the closure, which is effective immediately. It applies to all forms of contact with water, including fishing, boating, wading and using non-motorized crafts.

It's unclear how long it will take to clear the algae. Tammy Vigil, spokeswoman for the city's Department of Public Health and Environment, said it "will likely remain a problem for a while." Concerns about wildlife may also inhibit mitigation efforts.

"Significant rain and a cooler stretch of weather will help," she wrote in an email. "Also, use of algaecide on this type of bloom could knock the oxygen levels down so far that it would hurt aquatic life (fish and other aquatic organisms)." 

"The closure is temporary, and the lake will re-open when the algae bloom and cyanotoxin levels are shown to have subsided," city officials wrote in its announcement. Cyanotoxin is the toxin produced by the algae and affect the nervous system.

"Denver Public Health & Environment and Denver Parks and Recreation will continue to monitor the lake and provide updated guidance through the rest of the summer," according to the city statement.

The algae blooms are common in the summer and in areas with warm water, an increasing issue because of global climate change. Colorado Parks and Wildlife officials said earlier this month the agency was "concerned about increased levels of toxicity" in the wildlife area of Custer County. 

The Environmental Protection Agency wrote on its website that warm water, naturally and from climate change, can drive blue-green algae blooms because they "prefer warmer water. Warmer temperatures prevent water from mixing, allowing algae to grow thicker and faster." The agency went on to explain that "warmer water is easier for small organisms to move through and allows algae to float to the surface faster. Algal blooms absorb sunlight, making water even warmer and promoting more blooms."