Cedella Roman, 19, who is mixed race, was apprehended by US border patrol guards in White Rock, near Vancouver, after illegally entering the country.
She said she had not seen any signs marking the border as she ran along the beach before she went down a path to take a photo of the setting sun, when two male officers arrested her.
“[The border guard] started telling me that I had crossed the border illegally and I told him that I really did not do it on purpose,” she told Radio Canada.
She was threatened with a five-year ban from the US as the officers explained to her that they could not release her because she had been caught on surveillance cameras.
“I thought, maybe I’ve crossed the border, but maybe they’ll just fine me or they’ll say ‘come back to Canada’ or berate me a little bit,” she said.
Ms Roman had been in Canada since September 2017, spending time with her mother and learning English.
She did not have any identification on her when she was stopped, which further complicated her situation.
She was then transferred to a detention centre in Tacoma, a Department of Homeland Security holding facility, 125 miles south of where she first encountered the officers.
“They put me in the caged vehicles and brought me into their facility,” she said. “They asked me to remove all my belongings, as well as my jewellery; they searched me completely.
“That’s where I realised it was really serious and I started to cry a little.”
At the centre, Ms Roman rang her mother who brought her passport and study permits to prove her identity and right to be in Canada.
Staff at the centre, however, said the Canadian immigration authorities would have to validate the documents before she could be released, which led to a two-week stay at the facility.
“It was just unfair that there was nothing, no sign at the border,” said Christiane Ferne, Ms Roman’s mother. “It’s like a trap … anybody can be caught at the border like this.”
She was released on 6 June once authorities were satisfied her story was true.
A spokesperson for the US Border Services declined to comment on Ms Roman’s case but told Radio Canada that crossing the US border at a place other than a customs post was illegal.
“The regulation applies even if the person claims to have crossed the border accidentally,” the statement said.
Last year, 288 people were arrested for illegally entering the US from the Blaine area of Washington State.