The draining of canal locks at Apsley for restoration works laid bare the extent of rubbish that is dumped into the waterway.
The Canal and River Trust spends thousands of pounds each year clearing dumped waste, which can harm the wildlife and be a hazard to boaters.
People were invited into Hemel Hempstead’s Grand Union Canal locks 65 and 66 on Sunday after the water had been drained to allow maintenance of the lock gates.
Among the crayfish were shopping trolleys, tyres, a frying pan and other illegally flytipped rubbish.
Chris Stanley from the trust said: “It was all the usual things we expect to find. We did leave some of the rubbish in the bottom of the lock for people to see and they were amazed, so it was quite educational.”
The maintenance works are expected to take up to 10 days and the rubbish will be removed before water is allowed back into the locks.
Chris said: “Tyres and trollies can get lodged on the underside of boats or jammed behind locks.”
Flytippers run the risk of a hefty fine if caught.
