Caroline Lucas MP visited the River Waveney on the Norfolk/Suffolk border and said "beneath the surface all is not well".
Ms Lucas, whose constituency is Brighton Pavilion, said: "Bungay is lucky to have such a beautiful river teeming with wildlife, and it was lovely to see children swimming and playing on this hot summer's day.
"But beneath the surface all is not well. Dumping of raw sewage into the river has got to stop, and I call on Anglian Water to finally upgrade the sewers and consign this practice to history".
Residents showed her piles of wet wipes downstream of the pipe, which they said would have been flushed down toilets.
Toby Hammond, local Green Party organiser of a protest held in May, said more people would notice the issue in the summer when the river becomes busy with boaters, paddleboarders and swimmers.
"We've had rains on a couple of occasions in recent weeks, and after each one people have been sending me pictures of various unmentionable things emerging into the river," he said.
"What I find particularly outrageous is that only this week, Anglian Water, who are responsible for sewage treatment, have paid out £92m of profits to their corporate shareholders, yet we were told by the government last year that the improvements to ageing sewers were unaffordable."
"However, we agree that storm overflows are no longer an appropriate way of dealing with overloaded sewers during heavy rainfall, and we need to do more," a spokesperson said.
"We are already investing more than £200m to reduce storm spills between now and 2025, in places where it will have the most benefit to the environment."
The firm said it was committed to ensuring storm overflows and sewage treatment works "will not be the reason for unhealthy rivers by the end of the decade".