A friend she was with had fallen asleep. When she awoke, she discovered the woman was missing.
Her raft was found nearby and there was no evidence the woman had gotten out of the water, officials said.
MPD was called to the scene around 1:50pm.
They combed the area with assistance from the Manvel Volunteer Fire Department, the Brazoria County Sheriff's Office dive team, Pearland Fire Department and a Houston Police Department helicopter.
Texas Equusearch has offered their assistance in locating the woman, as well as the Montgomery County Precinct 1 dive team.
Their search will continue Monday, officials said.
The woman's name hasn't been released.