At every boat ramp around the three lakes, orange-vested Colorado Parks & Wildlife (CPW) employees stand guard against an insidious and almost invisible enemy: the tiny and prolific invasive bivalves known as zebra and quagga mussels.

On Monday, June 27, a boat arrived at the Stillwater boat ramp on Lake Granby with a number of half-inch long mussels attached to its hull. Inspector Fred Emst made the discovery. As inspectors prepare for the massive increase in boat traffic over the Fourth of July weekend, the catch was a sobering wake up call about the importance of their work.

“I’m doing my best to keep anything from getting in this lake,” said John Hall, a CPW inspector at Stillwater. “I want this to be here for my children’s grandchildren.”

The two species of mussel have been making a steady march west from their initial introduction to the Great Lakes in the 1980’s via ballast water from ships that had carried them across the Atlantic from Europe and Asia.

Mussels haven’t been found entering the three lakes since 2009, when a contaminated boat was caught entering Shadow Mountain Reservoir, according to CPW’s Brad Clements, who is in charge of field support for the 21 inspection sites in northwest Colorado.

They were first found in Colorado in 2008 when positive identifications of juvenile mussels were made in Pueblo Reservoir and Shadow Mountain Reservoir as well as seven other area water bodies, according to CPW’s invasive species coordinator Elizabeth Brown.

Shadow Mountain and the other water bodies were delisted after a new regional standard for what constituted an infestation was set in 2014. Pueblo remains listed as having a quagga mussel population, but Brown said CPW is hoping to delist it soon. Mussels haven’t been detected in Pueblo since 2011.

An adult mussel has never been identified in the state of Colorado, Brown said.

But the threat of invasion is growing. Surrounding states including Arizona, Nebraska, and South Dakota have no program in place to prevent the introduction of mussels.

“As these other states become infested, there’s more pressure on us to keep mussels out of the state,” Brown said.

Utah’s Lake Powell, a popular destination for Colorado boaters, is now the site of carpets of mussels on canyon walls and underwater structures. Brown and Clements both expressed concern about Colorado boats returning from Lake Powell and bringing mussels with them.

The mussel-infested boat that arrived in Grand County Monday had been purchased out of state and had not yet entered the water in Colorado. According to Colorado law, all vessels must pass an inspection before entering a body of water. It is also illegal to transport mussels between bodies of water in the state.

Once the mussels were identified, the boat in question was taken to Grand Lake’s East Inlet boat ramp where there is an open field large enough to conduct the decontamination process. Clements said that CPW then took care of the cleaning the boat, at no cost to the owner of the boat.

Each inspection site is equipped with a massive heated pressure washer that will kill and remove mussels, as well as other species of concern.

Brown said the interception at Stillwater was one of five interceptions of mussels this week and 13th this year, putting 2016 on pace to eclipse last year with the most interceptions in the inspection program’s history. There were 24 interceptions in 2015.

Monitoring lakes for mussel infestations involves trying to identify mussels at all three life stages, from the microscopic veliger stage to the “settler” stage where the organism develops into a two shelled organism, and finally to visible adult mussels. Brown said there was a dive team in Lake Granby Wednesday searching for evidence of mussels, but didn’t find any.

As thousands of boaters prepare to make the trek to Grand County for the Fourth of July, Clements and his inspectors are bracing themselves.

“It’s going to be chaotic this weekend in Grand Lake I’m afraid,” Clements said. Inspectors are on hand from 6 a.m. to 8 p.m. at inspection sites, but Brown said their days will likely go longer.

“You’re not putting a boat in this lake without me inspecting and decontaminating it,” Hall said. “It’s going to be crazy.”