Why can ONLY federal authorities regulate interstate commerce?

Question

The Constitution of the United States gives Congress the power to regulate interstate commerce. It seems the federal courts have held that only Congress, and not state legislatures, can regulate interstate commerce. In Sporhase v. Nebraska in 1982, the U.S. Supreme Court ruled that a Nebraska law forbidding commercial exportation of water from the state was invalid because of that.

I can imagine Congress passing a statute forbidding states to regulate interstate commerce, and that would be within their power to regulate interstate commerce. Did they? Or is there some other rationale for "only"?

Answer 1

The right belongs to the federal government because the Constitution says it does, in Article 1 (section 8, clause 3).

This, and the other rights listed in that section, are known as enumerated rights - that is, somebody has explicitly listed them out. The section begins

The Congress shall have power ..

and each clause may be read separately with that phrase prepended. They should each all be read together with the final clause, which is

• To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.

In the context of the Constitution, this enumeration means that Congress alone has the authority to make such laws. In particular, Congress is the only legislature with the right to make laws governing commerce between states and each other, states and Indian nations, or states and foreign nations.

Other sections and their clauses enumerate rights explicitly reserved to the States and the People, and further clarify that any rights not enumerated are also reserved thusly.

Answer 2

To answer your question generally, interstate commerce by definition involves multiple states. One state cannot pass laws that are binding on another because individual U.S. states are sovereign (with respect to each other -- they are still subject to the limits imposed by the federal Constitution), and therefore out of each others' jurisdiction to regulate. That is why interstate commerce is a federal matter exclusively -- it logically has to be. This logical inference is known as the Dormant Commerce Clause, and has been upheld by the Supreme Court in several notable cases (Gibbons v. Ogden, Zenith/Kremer Waste Sys. v. Western Lake Superior Sanitary Dist., and others).

Now, in the specific case of Sporhase v. Nebraska, there were two issues at stake:

1. The Sporhase farm straddled both sides of the Nebraska/Colorado border. Sporhase drilled a well on the Nebraska side, but used the water to irrigate his land on the Colorado side. Nebraska state law required an export permit to carry water out of the state, but the Director of Water Resources would not issue one unless that state also allowed exportation of their water to Nebraska. This was basically Nebraska trying to regulate Colorado's behavior with a carrot-and-stick law, and thus was ruled unconstitutional by the Supreme Court.
2. Water has this annoying tendency to flow wherever it wants and doesn't respect borders very much. Even though the well in question was in Nebraska, the underground aquifer it drew from ran across parts of 6 different states; Texas, New Mexico, Oklahoma, Kansas, Nebraska, and Colorado. Therefore, Nebraska didn't "own" that water any more than Ohio owns the Mississippi River.