Searches are set to resume for two people separately reported going into the River Taff on Tuesday.
Emergency services spent much of the day looking after a canoeist was seen in water near Taff's Well, north of Cardiff, shortly before 10:00 BST.
A man had also earlier been reported in the river at Taff Embankment in Cardiff city centre at 08:40 BST.
The fire service said it would be assisting with the search again on Wednesday.
Matt Jones, group manager at South Wales Fire and Rescue, told BBC Radio Wales Breakfast: "Extensive searches were carried out yesterday at both incidents and tragically no information has been found so far, but again we continue to search and today will see us continuing to support any activity that goes on to search these areas for signs of life or recovery of any evidence or equipment."
On Tuesday, police had appealed for anyone with a canoe who had been in the river near Taff's Well but had got out safely to contact them.
They also tweeted a photograph of equipment found during the search and asked if it was possible the owner could have been mistaken for someone in the water in difficulty earlier in the day.
Asked whether emergency services were certain the two people had actually been in the water, Mr Jones said: "Both incidents were taken on witness accounts. Members of the public have phoned in being concerned for life.
"We can't be 100% certain that, certainly with the River Taff incident with the canoeist, that someone was in the river.
"But we've done extensive searching with our partners and searches will continue again today.
"What we would urge is if there was someone was in the River Taff around the 9:45 time yesterday and they have safely got out of the River Taff, we'd urge them to contact the non-emergency police number to let us know they're safe."