LAS VEGAS (AP) -- Authorities took a boat onto Lake Mead on Tuesday to gather water samples they hope could shed light on mysterious brown foam found floating on the lake's surface over the weekend.

Park officials urged people to avoid the Overton Arm, a northern extension of Lake Mead, after several dozen carp were found dead and the foam was seen extending about eight miles from near the mouth of the Virgin River to Echo Bay.

The Southern Nevada Water Authority is monitoring water quality at two intakes and so far hasn't found anything problematic, according to spokesman Bronson Mack. Typically, pollutants are diluted in the reservoir.

"It really is a massive body of water, and that's one benefit from a drinking water perspective," Mack said, noting that water from the Overton Arm typically takes about a month to meander to the intake area.

A park volunteer collected water samples several days ago and they turned up normal, Mack said. But the water agency wants to gather new samples using more precise methods.

High winds and waves prevented crews from collecting water Monday, and the foam wasn't readily visible from the shore.

"We're hoping we can still get samples" of the foam, Mack said.

Lab tests on Tuesday's water collections are expected soon.

Mack said the Nevada Department of Wildlife is handling the investigation into the fish deaths.