Passage: Travelling to the Montana Territory, he became a trader and interpreter in the area surrounding Fort Benton. Observing rampant alcoholism and its detrimental impact on the Native American and Métis people, he engaged in an unsuccessful attempt to curtail the whisky trade. In 1881, he married Marguerite Monet dit Bellehumeur (1861–1886), a young Métis, "in the fashion of the country" on 28 April, an arrangement that was solemnized on 9 March 1882 at Carroll, Montana by Father Damiani. They were to have three children: Jean-Louis (1882–1908); Marie-Angélique (1883–1897); and a boy who was born and died on 21 October 1885, less than one month before Riel was hanged.Riel soon became involved in the politics of Montana, and in 1882, actively campaigned on behalf of the Republican Party. He brought a suit against a Democrat for rigging a vote, but was then himself accused of fraudulently inducing British subjects to take part in the election. In response, Riel applied for United States citizenship and was naturalized on 16 March 1883. With two young children, he had by 1884 settled down and was teaching school at the St. Peter's Jesuit mission in the Sun River district of Montana.

Question: In which district did Riel begin teaching?
Sun River district