The Ang Trapeang Thmor site boasts a long history dating back to the Angkorian era. Remains of an ancient laterite bridge are visible on the present-day main access road leading to the reservoir, which was part of a major causeway road. This road caused water to accumulate to the north of the causeway, contributing to the wetlands which are a key feature of the habitat today.
During Pol Pot's Democratic Kampuchea regime, an 11-km stretch of the east-west causeway was converted into a dam, along with a 9-km stretch of the north-south causeway. Both were undertaken using forced labour, and many villagers died from the strenuous work, malnutrition and poor conditions at the site. The structure was intended to provide irrigation and water storage for the cultivation of rice in the downstream area, but the structure was never completed and fell into disrepair.
The dam was renovated in 2004, and is now closed during the wet season to allow the build up of water to the north of the dam wall. This is then released in the dry season to irrigate the fields that are situated to the south of the dam. However, the exact timing and duration of the release of the water is a contentious decision each year; if too much water floods the area to the south, the rice crop there is destroyed, but if too little water is released, the build-up of water to the north destroys the crop on that side of the dam wall.