The Swan River's lower reaches were filled with a plume of silt yesterday, angering residents who fear the waterway has been polluted on the first day of the Fremantle Harbour dredging operation, the West Australian reported.

East Fremantle resident Leonie Anderson, a volunteer with the River Guardians scheme, criticised Fremantle Ports for dredging during yesterday morning's incoming tide, saying the operation had sent silt hundreds of metres upstream.

By lunchtime, the plume had travelled beyond Fremantle's two road bridges and was visible as a beige stripe in the centre of the river.

Anderson said it was frustrating because of moves this week to improve the river's health.

Fremantle Ports spokeswoman Ainslie de Vos said the plume was probably caused by "a combination of factors", including the departure of a large container ship, coupled with strong winds and tides.

"There may also have been some contribution from the dredging earlier in the day," she said.