"An unknown quantity" of milk flowed into the river, officials said.
ADVERTISEMENTA tanker carrying milk crashed into a river in west Wales on Wednesday, spilling its load and turning the water white.
The vehicle left the A482 in the Welsh country of Carmarthenshire.
Images posted on social media showed the River Dulais flowing white following the road traffic incident.
A video by May Lewis taken in Llanwrda showed milky water flowing through the village.
Natural Resouces Wales (NRW), which oversees the management of natural resources in Wales, explained in a Facebook post that the unusual sight was the result of a road traffic collision "that involved a milk tanker, which had left the road and entered the river".
"The tanker was carrying milk and an unknown quantity has escaped and entered the river. This has led to significant discolouration along the river," it said, adding that officers were assessing the impact.
Dyfed Powys Police confirmed just before 11 am that the road had been re-opened and the tanker removed, according to local media.