Dozens of people unable to take part in a charity zip slide in Bristol have been offered refunds or the chance to participate in an alternative event.
One hundred people were scheduled to take part in Bristol's Big Leap at Avon Gorge on Sunday.
But zip slide owners Outer Edge could only let 17 people use the slide as a boat passed through the gorge and due to additional technical difficulties.
Charity Cancer Research UK has been offered another event or a refund.
Outer Edge, which provided the zip slide, said the alternative could be held at the Sea Walls, or money paid to the firm by Cancer Research UK would be returned to the charity.
Lydia Johnson, who helped organise the event for Cancer Research UK, said volunteers had worked for a year to get it up-and-running.
She said people had paid a £40 registration fee and then had to raise a minimum of £160 to get a place.
Ms Johnson said the charity would give the money back, if the fundraisers wished.
Details on whether there would be an alternative zip slide would go on the Cancer Research UK website when a decision was made, she added.
She said: "People have raised a huge amount for charity and we are now faced with the position where of course we have to look at returning that money."
She advised people to check the Bristol's Big Leap website for refund details.
James Cook, who was among those signed up to take part, said: "As I was crossing the bridge they rang me and said there was going to be an hour, then a two hour delay.
"An hour after that phone call they rang me back and said I can't do it. They said the reason why was they had a boat go up the river."
John Paul Edgington, from Outer Edge, said a boat had unexpectedly passed through the gorge at 0630 BST, setting the event back by around 90 minutes.
He said: "The biggest delay was in transferring our braking system from the small test practices we've had and then scaling that up to something the size of the gorge.
"We did try and get a practice run at the gorge but trying to organise that was almost more difficult than organising the event itself. We've run practice runs at a smaller venue.
"It was going to be a really difficult event to pull off - it is 200m (656ft) wide, 90m (295ft) high and 250m (820ft) long. You can't go anywhere else and practice that.
"Our calculations said we can do this.
"I'm gutted to the core - I'm so sorry to all the participants and the organisers because of the amount of work that goes into these things."