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.
Sullivan Expedition happened in 1779.
Forbes Expedition happened in 1758.
Sicilian Expedition happened in -415.
Expedition Everest happened in 2006.
Narváez expedition happened in 1527.
Cassard expedition happened in 1712.
Expedition of Qatan happened in 625.
Villasur expedition happened in 1720.
Crawford expedition happened in 1782.
Jackson Expedition happened in 1863.
Expedition Amazon happened in 1983.
Nejd Expedition happened in 1818.