Tony Richardson, aged 70, of Mason Lathe Road, Shiregreen, was part of a group which found the explosive device in the River Don off Weirs Walk in Attercliffe.
The Blue Loop group, which clears Sheffield’s rivers and river banks, usually stumbles across shopping trollies, car tyres and old boots.
But this is one of the most interesting finds, coming as a huge shock to the team.
The group all passed around the device, thought to be a German World War One rifle grenade, and had no idea it could still go off.
At one point, volunteers stopped for tea and sandwiches with the unexploded device placed next to them on the ground.
Tony said: “Because it was so old, I think we all thought it was just an empty shell.
“We were pretty taken aback by it. A young lad in our group pulled it out of the river and then another member had hold of it and said it looked like an old grenade.
“He started rubbing the rust off it, none of us had any idea it was live, we were lucky really, it could’ve caused some serious harm.
“I had it in my hand walking around with the thing for about 10 minutes, then I called the police and later bomb disposal turned up.”
Tony had been at home for several hours when he got a phone call from a fellow volunteer who told him it had been detonated.
Tony said: “It takes a lot to shock me, and when I found out the thing was live, it really shocked me. It could have gone off at any time and a few of us had hold of it.”
Officers received the call around 11.25am and cordoned off the scene until Army bomb disposal units arrived on the scene around 2.20pm.
A South Yorkshire Police spokesman confirmed the device was detonated in a controlled explosion in an area of wasteland near the former Sheffield Ski Village site.