The latest government report noted a "general overall decline" in river pollution in the Isle of Man.
There were only 20 pollution incidents during the past 12 months, compared with 25 in 2010.
No major incidents were recorded in 2011, 18 were "minor" and two were "significant".
A government spokesman said the main problems were sewage and suspended solids.
He added: "Suspended solids such as dirt, silt, sand and dust, discolour the water and if present in sufficient quantities can smother fish eggs, fry and river insects.
"Sewage reduces general water quality and can kill fish and many sensitive insects which require very clean water to thrive."
The minor incidents of sewage pollution in 2011 happened at River Dhoo, Niarbyl Stream, Foxdale Stream and the River Glass.
The "significant", incidents involving suspended solids happened at Balleira Stream and Stoney Mountain stream.