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An algae infestation in Scofield Reservoir has bloomed to the point that local health officials might order the popular Carbon County destination closed Wednesday, just days before a busy holiday weekend.

The Utah Department of Environmental Quality has been monitoring a cyanobacteria colony on the 2,800-acre lake for much of August, but the bloom intensified over the weekend to a critical level and fish began dying. Thousands of rotting Utah chub and some dead bats lined the north and east shores and the water took on blue and green hues.

"We took more samples Monday because of significant changes in what we expected. We expected it to recede, but in the past week it really took off," said Brady Bradford, environmental health manager for the Southeast Utah Health Department, which will decide whether Scofield will be closed to boating, fishing and swimming.

Last Friday, the park upgraded an advisory on the lake to a warning, urging visitors to use caution if they come in contact with the water and to clean fish thoroughly.

This bloom comes at a time when Utah grapples with an unprecedented cyanobacteria crisis, largely in Utah Lake. These microscopic organisms feed on nutrient-laden runoff associated with agriculture and wastewater treatment. Compounding the problem this summer are warm temperatures and persistent low water that concentrates nutrient pollution and creates an optimal environment for algae to thrive. These organisms release a waste product that is toxic to humans and animals, and once the cyanobacteria die, their decomposition deoxygenates the water.

Whereas Utah Lake fills a valley bottom, Scofield sits at 7,600 feet above sea level in the headwaters of the Price River, a drinking water source for Price and Helper. The reservoir is surrounded by the Manti-La Sal National Forest on the Wasatch Plateau, long used for grazing. It has been considered an impaired body for phosphorous and low dissolved oxygen for at least a decade.

The reservoir also is a state park and is a favorite place for Utah County anglers to catch trout. The ongoing bloom is most intense in Scofield's narrow north arm near the dam and the Madsen Bay Boat Ramp. After five dry years, lake depth currently averages 12 to 15 feet, far below the 50 feet of wet years.

"That bay is pretty shallow. In drought years, it gets smaller and smaller — one of the reasons the levels are high is the natural wind will push materials toward the north end, and it doesn't have a large inlet. It just sits there," said Jonathan Hunt, manager of Scofield State Park.

Sampling from last week indicated cyanobacteria counts at about 5 million cells per millimeter at Madsen Bay and Frandsen Boy Scout Camp — well below a 10-million cell threshold for closing a body of water, but still alarming.

More samples were taken Monday, with results due Wednesday.

"If they come back high I can see us doing a closure — stay out of the water — and we might say don't use the boat ramps either," Hunt said. "The last thing you want is tell people it's safe and a crowd comes and it's not safe."

He noted that most state parks reservoirs are equipped with showers and urged park visitors to use these facilities since the cyanobacteria gets deposited in sandy shoreline where children like to play.

"You wash your fish off. Why not wash your kids off, too?" Hunt said.

Others high-elevation waters afflicted by algal blooms include Payson Lakes, a popular camping area on the Nebo Loop Road in Utah County. Warning signs are posted at Box and Big East lakes.

The Scofield fish die-off has mystified state wildlife officials because it did not affect trout, a species that is more vulnerable to low-oxygen events associated with algal blooms.

That suggests that the lake's dead chub did not succumb to low oxygen, according to Justin Hart, regional aquatics program manager for the Division of Wildlife Resources.

"Most of these dead fish are in big shallow bays. Trout would prefer not to be there because they are in the deeper areas," said Hart, who sees a silver lining in all those fish carcasses littering the shore.

"It's good news that we are not seeing any trout dying. It's just Utah chub, which we are trying to get rid of anyway through our management plan," he said.

bmaffly@sltrib.com Twitter: @brianmaffly