The name Spean Thma (pronounced "Speern Tmor") simply means "bridge of stone" in modern Khmer. The ancient structure is located just north of the Small Circuit Road, halfway from Thommanon to Ta Keo, but before the road crosses the Siem Reap river. The course of the Siem Reap river has shifted over the centuries, probably due to the sediment built up because of the bridge.
Spean Thma in its current appearance is a post-Angkorian construction. Stones from earlier temples were reused, this is indicated by carvings on some of them. The quality of the constructions lacks the preciseness of Angkor monuments. And the bridge was reconstructed several times. But there must have been a predecessor bridge already in the Angkor time. Spean Thma is the only noteworthy ancient Khmer bridge inside the Angkor Archaeological Zone. But there are many more - and much larger and more meticulous - Khmer bridges in other parts of the former Angkor empire. Spean Praptos with 25 arches at the road to Bang Melea is the longest of them. The builders used the same spanning techniques as for vaults inside temples. Examination of the construction of Spean Thma provided relevant information to modern researchers as to how the ancient Khmers constructed bridges. Furthermore, the remarkable difference between the former and present levels of the river bed at Spean Thma indicates a climate change and decreasing natural water supply, which may have contributed to the gradual decline of Angkor.
Spean Thma was constructed with 14 relatively narrow corbelled arches, only traces can be seen of most of them. They left only 1.1 metres wide passageways for the water, whereas the piers supporting the bridge are wider, 1.4 metres. This is one reason why the bridge was twice as wide as the river so that there is sufficient passage for the water to flow through. The other reason is annual flooding caused by heavy rainfalls during the monsoon season.
There is no optimal time of the day for a stop at Spean Thma. A ticket is not checked here. But using the Small Circuit road, you should have one.