Rivera "presented herself" at the border crossing at Gananoque, Ont., according to Carolyn Egan, the president of the Toronto area council of the Steelworkers Union, one of the groups that supported the soldier's efforts to stay in Canada.
The War Resisters Support Campaign said Rivera was "immediately arrested and detained, and transferred to military custody" and awaits transfer to a different military facility where she faces punishment for being absent from her unit. They have said she could face a court martial and time behind bars.
A U.S. Army private from Texas who served in Iraq in 2006, Rivera came to Canada in February 2007 to avoid being deployed again.
Rivera sought refugee status and had been living in Toronto with her husband and her four children, two of whom were born in in Canada. However, in late August, she lost her legal fight to remain in the country.
- Female war resister loses fight to stay in Canada
Rivera had previously said she would comply with the order to leave, despite some last-minute efforts to keep her in Canada.
The War Resisters Support Campaign organized a demonstration in Toronto Wednesday evening calling on Immigration Minister Jason Kenney to stop Rivera's deportation order.
Efforts to permit Rivera to stay here have also attracted some high-profile supporters, including Archbishop DesmondTutu
"The deportation order given to Ms. Rivera is unjust and must be challenged," Tutu wrote earlier this week in a commentary published in the Globe and Mail.
"It's in times when people are swept up in a frenzy of war that it's most important to listen to the quiet voices speaking the truth. Isn't it time we begin to redress the atrocity of this war by honouring those such as Ms. Rivera who had the courage to stand against it at such cost to themselves?"