Annaleise Carr of Norfolk County, Ontario, entered the lake at 6 p.m. in Niagara-on-the-Lake with 32 miles of water between her and her goal of reaching Marilyn Bell Park in Toronto Sunday, CTV News reported.
Annaleise had intended to start her swim Friday but delayed it a day because of high winds.
She is swimming to raise money for Camp Trillium, a recreational camp for children with cancer. Her Web site said she was within $1,000 of her $30,000 goal.
The teen trained for months to get ready for her long swim, CTV News said.
"Lake crossing looking like a go! Can't wait to touch that wall at Marilyn Bell Park!" she said in a Twitter post Saturday.