Wandle Park will be undergoing dredging from Tuesday 1 November to remove a build-up of silt in the reedbed channel.
The London Borough of Merton and Thames Water will carry out the work, which is expected to take about a week and a half. A small area of the park will be closed off to the public during this time.
The reedbed is responsible for cleaning surface water run off around Colliers Wood before it re enters the River Wandle.
Dredging has become necessary after Thames Water identified and stopped a number of illegal connections leading to pollution problems in the park that the reedbed has been unable to cope with.