Another son who had jumped in to help then became trapped by boulders and had to be rescued by his father.

The family were visiting Franconia Falls in New Hampshire on Tuesday afternoon, the US state's fish and game department said.

Located in the White Mountain National Forest, the picturesque waterfalls are a popular hiking destination and swimmers can slide off rock slabs into pools of water.

"One of the minor children slipped and fell into one of the pools at the falls," Sergeant Heidi Murphy said.

"He could not get out of the pool as it was a fast, circulating current. The mother jumped into the river to help her child and began to immediately have trouble."

Two other children jumped into the water to help their brother and mother.

"They were able to get their brother out of the water, but in doing so, another brother became lodged in the boulders and could not escape," Sgt Murphy said.

At the same time, the father was trying to find the mother. He eventually found her on a rock and immediately began CPR, but she was unable to be revived, Murphy said.

The father then made it over to his son's location and was able to pull him to safety.

Sgt Murphy said two people with non-life-threatening injuries were taken to a hospital for further evaluation and treatment.

The names of the mother and her family were not immediately released.