There has been a significant spillage of diesel fuel on the Plymouth coast.
The Environment Agency said it had put its emergency plan into place after the leak was reported at about 0730 BST, and said it was investigating.
The agency said the spillage into Victoria Wharf was thought to have occurred when a fishing boat was being refuelled.
The local Harbour Master said the worst areas had been dealt with and the clean-up was nearly finished.
Earlier Nigel Mayne, an eyewitness who was out on his boat at the time, said: "It's absolutely covered, you can see the red where the weeds are concentrated.
"The stink is quite horrendous."
It is not known exactly how much of the red diesel was released into the water, but the Environment Agency described it as a "large quantity".
The incident happened during an incoming tide and the fuel was pushed up the River Plym.
Tim Charlesworth, the Cattewater Harbour Master, who was in charge of the clear up, said: "While it's regrettable it's a fairly routine piece of harbour authority business to respond to oil spills.
"It is not a big spill - the effect on the harbour is not, in my view, dramatic.
"We've cleaned up the hotspots; beyond that it's a question of letting nature do its work."
Jon Snowden, from the Environment Agency, said: "We are in close contact with all concerned.
"Whilst we will assist where needed with the clean up, our efforts are turning to focus on the investigation of the spill and how it occurred."