Mormons expanded into the river valleys in northern Utah and southern Idaho in the 1860s. The growing population around Salt Lake City meant that people had to seek new lands for farms and villages. The main obstruction to northward migration was the presence of the Shoshoni Indians. Most of the Shoshoni were killed in a battle with government troops on the Bear River in 1863. After that, Mormons expanded unhindered into the Bear Lake and Malad Valleys. For many years, there were disputes about the boundary of Utah Territory. Many settlers thought they were in northern Utah while they had actually settled in southern Idaho. Primary sources.