Police have suspended a major search after two reports of people having gone into the water, including a capsized canoeist.
The canoeist was reported going into the River Taff at Taff's Well, north of Cardiff, just before 09:50 BST.
A search for a man was conducted on the river at Taff Embankment in Cardiff after a call out at 08:40 BST.
Specialist search teams are likely to resume on Wednesday, South Wales Police said.
The air ambulance and hazardous area rescue team were involved in the Taff's Well search.
People are being warned to stay away from rivers and coastal areas as Storm Francis caused havoc around Wales.
South Wales Police appealed for the canoeist to contact them if they had left the water safely and believed they were the person in question.
Officers also tweeted a photo of a wetsuit and other gear that were found in the search area, and asked if it was possible the owner could have been mistaken for someone in the water in difficulty earlier in the day.
#APPEAL | Do you recognise any of these items found by teams searching the #RiverTaff following reports of a capsized canoeist?— South Wales Police Cardiff (@SWPCardiff) August 25, 2020
Could you have left them behind? Could you have been mistaken for being in difficulty in the water earlier this morning?
📞101, ref 2000310377
The force said it had also received a report of a man entering the water near the Principality Stadium.
A multi-agency team had been involved in both searches.
"Due to the adverse weather conditions, searches of the water have been stood down for the day, with specialist search teams likely to resume tomorrow," said a police spokesperson.
"Searches of the riverbanks and the vicinity of both incidents will continue this evening."
Meanwhile, in a separate incident, a woman has been rescued from the River Ely in Leckwith, Cardiff and has been taken to safety, police said.
Penarth Lifeboat Station said it was believed the woman, who was rescued along with a dog, had gone into the water to help the animal.
It said they were rescued by a harbour master vessel.
"Although obviously cold and wet both appeared in good spirits and were taken for medical checks," said the lifeboat station in a Facebook message.