The woman had been part of an organised trip on the river and was traversing a section of the river known as “the wallet” when she fell, according to local news outlet Le Dauphine.
She has not been named, either in local media or by authorities in France or the UK.
An “involuntary manslaughter” investigation had been opened and an autopsy will be conducted, Lozac’hmeur also said.
Last year, a British man died after getting into difficulty on the river Durance, which also passes through Briançon.
He had been part of a kayaking group in June 2024 after a period of heavy snowmelt upstream, local newspaper Le Dauphine reported. Two others were rescued uninjured.
Both the Guisane and Durance are popular rivers for white water rafting, fed by glaciers and snowmelt in the spring.
They have sections of challenging rapids, but are usually only navigable for two or three months in early summer, according to Briançon’s official website.