One person suffered serious injuries and five others were lightly injured when rocks hit two groups of hikers on the right bank of the lake just after 1pm, Bern police said in a statement on Monday.
Two teenagers and four adults were airlifted to hospital, while an injured dog was treated by a vet.
The affected path has been cordoned off while geologists investigate the cause of the rockfall. Operation of the Gelmerbahn cable car up to the lake was also temporarily suspended, but later reopened.
Photo: Bern police
Witnesses to the rockfall told newspaper 20 Minuten that around a hundred people were hiking around the lake at the time.
One reader, who was on the opposite side of the lake when the rockfall occurred, said it lasted for around 20 seconds.
“The noise was deafening,” he said. “Then we heard the screams of the wounded.”
The Gelmersee reservoir is a popular hiking spot in the Grimsel region, accessible via the Gelmerbahn, the steepest funicular in the world.