<a href="http://en.wikipedia.org/wiki/Khwae_Yai_River" rel="noreferrer nofollow">The Khwae Yai River</a>, also known as the Si Sawat, is a river in western Thailand. It has its source in the Tenasserim Hills and flows for about 380 kilometres through Sangkhla Buri, Si Sawat, and Mueang Districts of Kanchanaburi Province, where it merges with the Khwae Noi to form the Mae Klong River at Pak Phraek subdistrict.

The famous bridge of the Burma Railway crosses the river at Tha Makham Subdistrict of the Mueang District. However this is not the same bridge as depicted in The Bridge over the River Kwai by Pierre Boulle and in its film adaptation. A bridge was built of wood approximately 100 metres upriver from the current bridge, during the construction of the iron and concrete bridge and also rebuilt in 1945 when the iron bridge was bombed.

No remnants of the wooden bridge remain. That wooden bridge was also not the bridge depicted in the film as the river was not called the Kwai Yai at that time. A wooden trestle bridge was built over the Kwai Noi many miles upstream in the jungle and it would more closely resemble the bridge in the film. However, the film is really a fictional depiction of the events with many inaccuracies and neither bridge can really be said to be that depicted in the film.