Around 130,000 gallons of slurry has spilled into the Afon Teifi in Cenarth, Carmarthenshire, and a small nearby tributary.
Environment Agency Wales attended the scene yesterday afternoon after a farmer alerted officers to the leak in his slurry store.
So far there have been no reports of dead fish in either river.
After inspecting the rivers this morning, officers are reporting that the high river flows have quickly dispersed the pollution and there is little remaining evidence of the slurry.
The small tributary remains slightly discoloured, but officers expect this to clear shortly.
An agency spokesman said: “We’re thankful to the farmer for reporting this incident to us, and we will continue to monitor the area over the coming days to check that there are no further impacts on water quality in the rivers.
“We’re also advising the farmer on how to conduct a cleanup operation on the farm without causing any more slurry to enter the rivers.
“If anyone notices any further signs of pollution in the area they should report it directly to us through our incident hotline on 0800 80 70 60.”
As a precaution, the Dwr Cymru drinking water abstraction point at Llechryd was temporarily closed for a short period last night. This has since been reopened as the incident has not impacted on drinking water quality.