By Lesley Wroughton

TAN AN TAY VILLAGE, Vietnam (Reuters) - As John Kerry's boat winds its way along the turbid waters of the Cai Nuoc river in Vietnam's Mekong Delta, the U.S. Secretary of State is taken back to the time he spent here as a commander of an American patrol boat during the Vietnam War.

But it is not only the past that brings Kerry to this remote part of the Mekong, the 12th largest river in the world. He is here to deliver a message about the growing threat of climate change and the impact it will have on the delta and the millions of people that depend on it for food, water and transportation.

It is the first time that Kerry has returned to the Mekong since 1968 when he served as a young U.S. naval officer in Vietnam battling Viet Cong guerillas in a conflict that earned him three Purple Hearts, a Silver Star and a Bronze Star.

"It is obviously amazing for me to be here today," Kerry told students gathered on the banks of the Cai Nuoc river. "Decades ago, on these very waters, I was one of many who witnessed the difficult period in our shared history," he said.

"Today on these waters I am bearing witness to how far our nations have come together and we are talking about the future. That is the way it ought to be," he said. "As our shared journey continues, our eyes are firmly fixed on the future and not on the past."

Dressed in casual khaki-green trousers, a blue-checked shirt and wearing sunglasses, Kerry is surrounded by aides and other officials on the boat but he is mostly quiet and introspective.

"It hasn't changed all that much," Kerry remarks at one point during the tour. The familiar smell of burning firewood in the air coming from villages takes him back to his time on the river.

At the small riverside community of Kien Vang, translated roughly as "the golden ant", Kerry stops to take a walk, visiting a small convenience store where he buys candy for the local children. There he pets a dog and is reminded of a mutt he adopted while serving in Vietnam he named "VC" - short for Viet Cong.

Here he also inquires from Dang Kieu Nhan, deputy director of the Mekong Delta Development Research Institute, about water levels and how possible changes in water flows on the river will affect villagers.

His concern is not only the effects of climate change on the Mekong but also plans by China to build four more dams along the Mekong to generate power for its rising economy, projects that will have a disastrous downstream impact on Cambodia and Vietnam, according to environmental specialists.

In addition Laos is also proposing to build hydropower plants on the Mekong, while Cambodia has plants for two dams on the river.

Across the canal, Kerry addresses these developments in a speech to the students, while also pledging $17 million to a program to address the impact from potential climate changes.

"No one country has the right to deprive another country of the livelihood and eco-system and its capacity for life itself that comes with that river," Kerry says.

"That river is a global asset, a treasure that belongs to the region, and so it is vital that we avoid dramatic changes in the water flow and sediment levels. Already we are seeing fisheries are experiencing threats to the fish stocks as a consequence of the changes taking place," he adds.

Kerry says he will raise the issue when he next visits China "so that we can work together on it in an effective way."

(Reporting by Lesley Wroughton; Editing by Sandra Maler)