Tonle Srepok - "Apocalypse Now" was likely based on this Mekong tributary.

[CEOCambodiaNews]

Tonle Srepok
The river depicted in the antiwar film "Apocalypse Now" was likely based on this Mekong tributary.
28 November 2018
Tonle Srepok is a major tributary of the Mekong River that runs between the Vietnamese province of Đắk Lắk, and the Ratanakiri and Stung Treng provinces in Cambodia. Before the development of modern roads, the river was crucially important to Lao, Khmer, and Vietnamese people for trading purposes, and as a supply line during the U.S.-Vietnam War.

Tonle Srepok (also called the Srepok or Serepok River) was along a branch of the Ho Chi Minh Trail and was used to supply North Vietnamese forces fighting in South Vietnam with weapons, ammunition, and food. When the U.S. Army discovered this, the whole area was heavily bombed, leaving behind a huge amount of unexploded ordnance that even today maims those unfortunate enough to step on or hit one. Rumors of U.S. Special Forces infiltrating Ratanakiri province abound—as to the veracity of these rumors, opinions vary.

Know Before You Go

If you want to see TonleSrepok before it merges with other rivers, the best place is Lumphat. Alternatively, Road no. 76 runs south from Lumphat, across the Lumphat Wildlife Sanctuary, and along Tonle Srepok, and for the first couple of kilometers, this road is about 2 km from the river. Further south, the road diverges to a distance of about 15 km, and having a guide vastly increases the chance of finding the river, and more importantly, finding the way back to the road. Due to the presence of unexploded ordnance, you should not stray off well-trodden paths.
https://www.atlasobscura.com/places/tonle-srepok

[Barang_doa_slae]

While biking from Mondolkiri to Reatanakiri in the early 00’s I had the dumb idea to make the trip even harder by going east and trying to cross the river near Lumphat, the idea was to follow Hoh Chi Minh Trail as seen on the USAF maps we were traveling with.
Long story short, we backtracked only few miles from the river after what I still believe today was a ghostly white lady told us not to cross into the forest standing ahead of us.