The Mekong Delta is the region in southwestern Vietnam where the Mekong River approaches and empties into the sea through a network of distributaries. The Mekong Delta region encompasses a large portion of southwestern Vietnam of over 40,500 square kilometres (15,600 sq mi).

The size of the area covered by water depends on the season. Before 1975, Mekong Delta is part of the Republic of Vietnam. Mekong Delta is home of the IV Corps region during the Vietnam War. IV Corps is the only corps in South Vietnam that VC didn't attack significantly until the last President Duong Van Minh surrender to North Vietnam.

Today, the region comprises 12 provinces. The Mekong Delta has been dubbed as a "biological treasure trove". Over 1,000 animal species were recorded between 1997 and 2007 and new species of plants, fish, lizards, and mammals have been discovered in previously unexplored areas, including the Laotian rock rat, thought to be extinct.

More Info: en.wikipedia.org