SACRAMENTO, Calif.—A Northern California family found dead on a hiking trail near the Merced River died after they overheated and ran out of drinking water on a sunny August afternoon when temperatures reached 109 degrees Fahrenheit in the steep mountain terrain, authorities said Thursday. The deaths of Jonathan Gerrish, his wife, Ellen Chung, their 1-year-old daughter, Aurelia “Miju” Chung-Gerrish, and their dog, Oski, had baffled investigators. The case involved more than 30 law enforcement agencies who had painstakingly reviewed—and ruled out—causes such as murder, lightning strikes, poisoning, illegal drugs and suicide. On Thursday, Mariposa County Sheriff Jeremy Briese said investigators concluded the family died from hyperthermia, a condition caused when a person’s body temperature is dangerously high after exposure to hot, humid weather. It’s unclear what killed the dog, an 8-year-old Australian shepherd and Akita mix. But Briese said evidence indicates the dog was “possibly suffering from heat-related issues.” “This …