A river that turned white after a lorry tanker spilled thousands of litres of milk into it is now running clear.
A tanker crashed into the River Dulais near Llanwrda in Carmarthenshire after it left the A482 road between the A40 and Lampeter on Wednesday at 12:30 BST.
Dyfed-Powys Police said the male driver of the tanker was not seriously hurt and they are not investigating.
Natural Resources Wales (NRW) said 28,000 litres of milk leaked, causing "significant discolouration".
NRW has now said the River Dulais, which runs into the Tywi, is now running clear.
BBC presenter and environmentalist Will Millard said while milk may appear a benign substance we drink every day, once it gets into the watercourse, it can affect hundreds of miles and become the equivalent of "napalming a forest".
He said it had the potential to "extract oxygen at a terrific rate", affecting the "complex web of life" that includes zooplankton which fish eat, plant life which voles are reliant on and the fish that birds feed on.
"If you kill one thing, it can quickly kill the whole river," he said.
"Rivers are one of our most vulnerable eco-systems, it doesn't take much to wipe them out completely.
"The good news is, they are very robust and can bounce back."
He urged anyone who lives around the River Dulais to "keep their eyes on the water" and report any dead fish, so any further issues arising from the spill can be dealt with quickly.
Authorities are checking the river downstream from the incident, near to the where it runs into the Tywi, which is about a mile down the valley.
The lorry spilled milk after it crashed down a ravine and into the River Dulais after travelling northbound between Llanwrda and Pumsaint near the Feathers Caravan Park.
Police shut the A482 road for more than 10 hours so Carmarthenshire council could recover of the overturned tanker, which had partially blocked the carriageway.
The Welsh Ambulance Service said they did not attend the scene.
Ioan Williams of NRW said they are continuing to closely monitor the situation as they said milk is up to seven times more polluting than cattle slurry.
The river is now running clear where the tanker overturned, but a milk 'slug' is reported to be travelling slowly downstream in the River Tywi, and was last seen near Llandeilo.
NWR biologists are planning to monitor the insects and invertebrates along the waterway over the weekend, but are still asking anyone who spots any dead fish to get in touch.