Final answer:
The northeastern-most border of Kansas is bordered by the Missouri River, which was a significant geographic and symbolic boundary during the mid-19th century conflicts over slavery in the region.
Step-by-step explanation:
The river that borders the northeastern-most border of Kansas is the Missouri River. This river has played a significant role in Kansas history, especially during the period of antebellum America when Kansas was a hotspot for pro- and antislavery activism. During the 1850s, supporters and opponents of slavery flooded into Kansas, with Free-Soilers endeavoring to prevent the expansion of slavery into the territory and proslavery activists looking to secure Kansas as a slave-holding region. The Missouri River served as a natural boundary during these events.
It's important to note that the geopolitics and physical geography of the area interacted closely, as the Missouri River was not just a physical barrier but also a symbolic one between the proslavery state of Missouri and the contested Kansas territory. The Missouri River itself flows eastward and connects with the Mississippi River, further emphasizing its prominence in shaping the political and social landscape of the region during the mid-19th century.