A Whanganui East resident found one living freshwater mussel as he pulled smelly plastic bags and other rubbish out of the Matarawa Stream.

Larn Sweeney has lived in his Raine St house for four years. Having the Matarawa Stream run through its back garden was one of its good points.

But he has pulled three trailer-loads of rubbish out of the stream so far and is now finding a layer of plastic bags submerged in the mud in its bed. That would make it a very tough place for the sole remaining freshwater mussel (kākahi) he found.

Kākahi used to be common in the soft, sandy sediment of New Zealand streams, lakes and rivers. They burrow into soft sediment and filter water - having a major cleaning effect where they are found in good numbers.

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They're under threat and declining, perhaps partly because they start their lives as parasites attached to kōaro, a native fish. Kōaro are one of the whitebait species, and also in decline.

"Whitebait don't get up this far, because the people down in Kowhai Park block the stream. In four years of looking I have never seen a whitebait get up this far," Sweeney said.

He's clearing the stream to stop rubbish blocking it and creating a dam. It came near to flooding his house in June 2015.

He said people used to treat the stream like a tip. He's found car batteries and difs, tyres, and bits of washing machines in it.

There's less hard rubbish being put in now, but people are still using it to take away their garden rubbish - weeds like convolvulus, and fresh prunings.

But things are improving. Sweeney's upstream neighbour is also pulling rubbish out, and he's noticed some people are even taking down the fences that separate their sections from the water, and planting trees.

Horizons Regional Council cleared half of the urban stretch of the stream last summer, at a cost of $320,000 to Whanganui ratepayers. It will work on the other half this summer.

Sweeney said they're doing a good job, from what he can see, and others could be helping out.

"I want the bed to be free of rubbish so that if someone does walk up there with bare feet they're not going to end up with a bottle in their foot."