Published: Aug 16, 2011 4:51 PM EDT

KLAMUTH, Calif. (AP) - A 45-foot gray whale that's been stranded in Northern California's Klamath (KLA'-muth) River for more than a month has died after beaching on a sandbar.
    
Sarah Wilkin of the National Oceanic and Atmospheric Association says the female whale died around 4 a.m. Tuesday morning. Wilkin was part of the team studying the whale and urging it back to the Pacific.
    
Wilkin says a cause of death will be determined in a necropsy after researchers move the marine mammal.
    
The mother gray whale and her 15-foot calf entered the river in late June during their northward journey from breeding grounds in Baja California up to Alaska.
    
Rescuers spotted what they believed to be the calf swimming back to sea on July 23 after weaning from the mother.

(Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)