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Saving our reservoirs from invading ‘cling-ons’

Vigilant inspections keep destructive mussels from causing millions in damage to Denver’s water infrastructure.

Aquatic nuisance species attached to the bottom of a boat found during an inspection at Dillon Reservoir. The boat was barred from entering the reservoir to protect water supply infrastructure. Photo credit: Dillon Marina.


Like a page out of a science fiction novel, a wave of aquatic invaders is trying to discretely slip into the nation’s reservoirs and lakes.

They’re hiding on boats and hitchhiking from one infected waterway to attack the next.

The main culprits are two varieties of freshwater shellfish, the zebra and quagga mussel, considered to be aquatic nuisance species.

Since the late 1980s, when these mussels were discovered in the Great Lakes region, these intruders have spread to more than 35 states, mostly by boat, which can easily transport these aquatic hitchhikers.

In fact, this exact situation played out at the Dillon Reservoir marina in early August.

A pontoon boat that had visited Lake Erie only days before its arrival at the shores of Dillon Reservoir was flagged on inspection before it could enter Denver Water’s largest reservoir. Inspectors found the boat to be infested with aquatic nuisance species.

Aquatic nuisance species, such as those found on this boat during an inspection at Dillon Reservoir, can cause millions in dollars in damage to water infrastructure. This boat was barred from entering the reservoir. Photo credit: Dillon Marina.


“It just takes a few microscopic larvae entering the reservoir, or adults attached to a boat, and before you know it these mussels can overtake dams, valves and pipes, causing millions of dollars of damage to critical drinking water infrastructure,” said Brandon Ransom, Denver Water’s manager of recreation.

Denver Water opens four reservoirs to motorized boating around Memorial Day every year and has helped pay for inspections to keep the invaders at bay since 2008.

That work has paid off. No adult mussels have ever been found in Colorado.

Pueblo Reservoir tested positive for zebra mussel larvae in 2007 and quagga mussel larvae in 2008. There has not been a detection in Pueblo for five years, and the reservoir has been removed from the Colorado Parks and Wildlife’s “known positive waters” list.

Since 2008, Denver Water has paid about $200,000 a year to help fund the boat inspections at Antero, Williams Fork and Eleven Mile Canyon reservoirs.

At Dillon Reservoir, one of the more popular recreation destinations in the state, Denver Water pays $70,000 a year for the two marinas to perform the inspections. In 2019, Dillon and Frisco Bay marinas performed more than 1,600 total inspections for aquatic nuisance species.

New regulations passed in January 2017  are designed to help prevent the spread of aquatic nuisance species, such as the ones found on this boat during an inspection at Dillon Reservoir. Photo credit: Dillon Marina.


So, how do you keep these invaders off your boat? Colorado Parks and Wildlife has a simple strategy for all boaters to follow each time they exit a body of water: Clean, drain and dry.

As an added safeguard, state regulations passed in January 2017 require boaters to remove aquatic plants and water drain plugs before leaving a parking area after being on the water. It is now against the law to transport a boat and trailer over land with drain plugs in place or vegetation attached to the vessel. Boaters who don’t comply with this rule can be ticketed.

“We want people to come to these boating reservoirs for a fun day on the water,” said Ransom. “We just ask that visitors remain mindful of how critical these waterways are to Denver Water’s drinking water system and continue to be responsible stewards of these facilities.”

If you’re planning on boating on any of Colorado’s 200 lakes and reservoirs this year, check out all of the aquatic nuisance species regulations on the Colorado Parks and Wildlife website.