A dead cat and bags of rubbish were among the items polluting the River Wandle, causing a flood risk.
The not-so-lucky black feline was spotted floating in the river by the Weir Road Estate in Wimbledon by Mark Stephenson, who works at nearby Affordable Architects.
It was near bags of rubbish and a shopping trolley abandoned in the river on the Wandsworth border by Plough Lane.
The Environment Agency (EA) said the debris could cause a flood risk, and removed it soon after being contacted by the Wimbledon Guardian and Mr Stephenson.
Mr Stephenson contacted the EA and Merton Council to alert them to the rubbish he said is collecting in the river.
Mr Stephenson said: "There’s more and more rubbish being dumped every day.
"It is collecting under the bridge.
"It is building up.
"The river is dying- there’s a dead cat on top of it."
The council said if the litter was in the river itself, it was the EA’s responsibility to clear it.
An EA spokesman said it sent members of staff to clear the rubbish the day after the Wimbledon Guardian contacted them.
The spokesman said: "Our operations team are going to the site to clear it up as we believe the rubbish could pose a flood risk.
"We thank the person for reporting this incident to us so that we can take action."