SPOKANE -- A combination of fatigue and Milfoil is what ended up killing a 23-year-old Elk woman who drowned Wednesday on Horseshoe Lake in southern Pend Oreille County.

The Pend Oreille County Sheriff's Office say 23-year-old Rachel Wood went with family to the remote lake. Wood decided to swim with a group of people across the lake to a cascading waterfall, a popular destination for visitors.

"The waterfall back there there's some rocks you can jump off,” boater John Alexander said.

One kayaker at the lake Thursday estimated it’s a two-mile journey. On the way to the falls, the group got into trouble.

The sheriff's office says Wood got tired and entangled in Milfoil. She was trapped underwater for anywhere from 10 to 20 minutes.

A boater was flagged down and helped pull Wood out of the water until emergency crews arrived. Wood was flown by Medstar to Sacred Heart Medical Center where she died.

John Alexander and his wife Crystal frequent the lake and say Milfoil can be overwhelming especially for any tired swimmer.

“It’s not a lot if you stay away from the edges but of course, if you're trying to swim to shore and you're tired you'll go right into it,” Alexander said.

“It’s hard to keep your strength because it does surround you."

Experts say Milfoil isn't always dangerous so much as how swimmers react when they get into it. They recommend you make a water plan, know your true swimming ability, wear a life jacket and if you get entangled to stay calm.