The Songkhram rises quietly in the hills between Nong Han district in Udon Thani province and Sawang Daen Din district in Sakon Nakhon, in country that gives no hint of what the river becomes downstream. From there it wanders northeast and east through Seka district in Nong Khai province, through Wanon Niwat back in Sakon Nakhon, and into Si Songkhram district - the town that took the river's name and built a fish trade on it. Along the way it picks up the smaller Oun, and together they drain the upland, forest-covered northern third of Nakhon Phanom province - country with a rougher character than the flat paddy plain further south. It finally reaches the Mekong at tambon Chai Buri in Tha Uthen district, joining the larger river from the west. The whole course stays within Isan. It is one of the more important rivers in Thailand's northeast and one of the least known outside it.

What makes the Songkhram unusual is what it does not have. No dam, no barrage, nothing across the channel to interrupt the flow - the only Mekong tributary in Thailand left in that condition. The consequence arrives every year with the floods. Fish swim up out of the Mekong into the Songkhram to spawn, unimpeded, and the young fish that result restock a stretch of river system that feeds a very large number of people. The lower basin supports 192 species of fish, 136 species of birds and 208 species of plants. For households along the banks, that is not biodiversity in the abstract. It is dinner, and it is income, and it depends entirely on a fish being able to swim from one river into another without meeting a wall.

In 2019 the lower Songkhram basin was designated a Ramsar site - a wetland of international importance - the fifteenth in Thailand. The listing covers a 92-kilometre reach of the river along with 34,000 rai of surrounding basin, some 5,400 hectares, and it arrived through an unlikely coalition: local district governments, the National Environmental Policy Office, WWF-Thailand and a commercial bank. Roughly 240,000 people across 49 communities live inside the area the designation is meant to protect. Ramsar status is not a legal shield in itself. What it does is put the river on an international register, where anyone proposing to change it has to explain themselves to a wider audience than the provincial governor.

The proposals keep coming. Government bodies and private investors have been pushing for a dam on the Songkhram since the 1980s, and every scheme so far has been beaten back by the villagers, fishers and civil-society groups who would live with the result. In 2019 the Royal Irrigation Department returned with a watergate planned for Tha Uthen district in Nakhon Phanom, near the river's mouth, as part of the sprawling Khong-Loei-Chi-Mun water-diversion project. The argument has not changed much in forty years: engineered water storage against a free-running fishery, calculable megalitres against a food supply nobody has ever properly priced. The people who have won this fight repeatedly are not professional campaigners. They are households who know exactly what the river is worth because they eat it.