Tap above to watch video report by Martin Stew
Just to the east of Brent, Welsh Harp Reservoir is a site of special scientific interest - the birds who breed there and plants on the shore are each deemed worthy of protection. But along the banks now, all you can see is rubbish.
Over the last five years a team of volunteers has collected 2,000 bags of rubbish at the reservoir.
And after the levels were lowered a few weeks ago for maintenance it's clear there is plenty more dumped waste.
"It's so disgusting... we've got tonnes and tonnes of rubbish, plastic, cans, polystyrene - an endless amount of detritus," said Daniella Levene of Friends of the Welsh Harp.
"It makes me cry quite a lot. Especially as we have been going at it for five years and it is still no better," she added.
During one weekend the group collected 150 bags of rubbish.
"I fluctuate from feeling really heartbroken because this is a really important nesting site for birds and feeling mild rage because it's quite simple - there is rubbish here which needs to be cleared," said Leila Taheri from Friends of the Welsh Harp.
Leila said the problem was so bad that volunteers alone could not fix it.
"A few organisations own this land. The Canal and River Trust look after the water and are responsible for clearing it. The Environment Agency is responsible for clearing the trash traps that feed into this reservoir," Leila added.
The Canal and River Trust said it was "equally frustrated" by the amount of litter.
"We are committed to making sure we put a plan together with our partners to make sure this gets resolved," said Ros Daniels of the Canal and River Trust.
The Environment Agency said it was aware of, and would clear, large amounts of debris. In the meantime litter pickers are warned to take care because silt banks can be as dangerous for people as the wildlife is for rubbish.