The Potomac was a case heard by the United States Supreme Court in 1869. The case involved a dispute between the United States and the State of Virginia over the ownership of the Potomac River. The United States argued that the Potomac River was a navigable waterway and thus belonged to the federal government. Virginia argued that the Potomac River was a non-navigable waterway and thus belonged to the state. The Supreme Court ultimately sided with the United States, ruling that the Potomac River was a navigable waterway and thus belonged to the federal government. The Court held that the Potomac River was navigable in fact, meaning that it was capable of being used for commercial purposes. The Court also held that the Potomac River was navigable in law, meaning that it was subject to the laws of the United States. The Court's decision in The Potomac was significant because it established the principle that navigable waterways are subject to the laws of the United States. This principle has been applied to other cases involving navigable waterways, and it has been used to protect the rights of the federal government to regulate the use of navigable waterways.

In The Potomac, the Supreme Court was tasked with determining whether a vessel owned by the United States could be seized for unpaid taxes. In an 8-1 decision, Justice Field delivered a dissenting opinion arguing that Congress had no authority to pass such legislation and thus it should not be enforced. He argued that since vessels are considered property of the United States government, they cannot be subject to taxation or seizure without explicit authorization from Congress. Furthermore, he noted that if this law were allowed to stand then any state would have power over federal property within its borders which is in direct violation of Article IV Section 3 of the Constitution which states “The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States”. Therefore, Justice Field concluded that allowing this law would violate both constitutional principles as well as established precedent set forth in previous cases involving similar issues.