Rivers can become waterfalls; wrecking your boats, destroying your equipment and supplies, and/or killing your crew.
Unfriendly natives could kill your crew, steal your supplies, or just misdirect you into dangerous situations.
Unknown illnesses could wreak havoc with your crew's abilities to continue, or it could kill them outright.
Crawford expedition happened in 1782.
Expedition of Qatan happened in 625.
Jackson Expedition happened in 1863.
Lamellerie's expedition happened in 1806.
Villasur expedition happened in 1720.
Expedition Amazon happened in 1983.
Hidden Expedition happened in 2007.
Romney Expedition happened in 1862.
Expedition of the Thousand happened in 1860.
Falklands Expedition happened in 1831.
Seymour Expedition happened in 1900.
Expedition Everest happened in 2006.