He said as there was no phone signal in the gorge and he could not make contact with his friends.
Mr Weaver said he spotted them with a German couple and two Spanish nationals in a “bottleneck” between two boulders.
“Water [then] appeared to come from nowhere. The sound was incredibly loud, people were screaming.”
Mr Weaver said he could see one of the group had managed to grab on to a rock while one of the Spanish nationals had been swept into the torrent and was being carried away.
His own partner managed to get out of the water and, after a few minutes, others emerged from the deluge.
But Mr Weaver said the noise of the water was too loud for them to communicate.
Ms Thompson was seen being swept away along with Mr Barrett, who had tried to help before the current took him too.
Survivors who had suffered minor injuries were taken out of the gorge by helicopter.
Jyoti Gill, assistant coroner for Manchester South, recorded a verdict of accidental death caused by drowning for both Ms Thompson and Mr Barrett.
“Sadly, both Sarah and Alex were unable to climb to safety and were swept away by the floodwaters,” Ms Gill said.