The National Park Service says the family entered the park at the Ash Mountain entrance gate at about 6:45 a.m. and walked down to the Kaweah River near the entrance sign.
The 5-year-old boy slipped into the river, and several people jumped in to help including the parents and the 22-year-old who was with the family.
Three fishermen who were on the shore helped the parents and child out of the water, but could not get to the 22-year-old.
The father and one of the fishermen began CPR on the boy and were able to resuscitate him.
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At about 7:15 am, Park Rangers were told what happened and they immediately began to search the river with the help of Tulare County firefighters and Tulare County Sheriff's deputies.
At the same time, the young boy was rushed away by ambulance to meet up with a helicopter that took him to Kaweah Delta Medical Center in Visalia. The boy has since then been transported to another hospital where his treatment will continue.
About two hours after he went into the water, search crews found the body of the 22-year-old.
This is the park's second river fatality within two weeks, and both were similar incidents in the same location.
"In both cases, the family members who jumped in after the children did not survive," Sequoia District Ranger Dave Fox said. "The rocks are super slick on the river's edge, and people should avoid getting close to the water."