Can the US Congress or Executive unilaterally change interstate compacts after they're established?

Question

There are some notable compacts such as the Great Lakes compact, Atlantic Fisheries compact, etc… that seem to require a large amount of authority to enforce in practice.

Theoretically it makes sense for such compacts to be reliant on the sovereignty of the states themselves, which would suggest that they can only be altered based on mutual consent of the State parties.

Though on the other hand, why would the federal government allow such an organization that can’t be modified or eliminated in the future at the federal level (without a court order), to be established in the first place under their aegis? (As all compacts have to be authorized by Congress before coming into effect)

Answer 1

A duly enacted federal statute or treaty passed after the interstate compact is entered into may supersede or abrogate provisions of the interstate compact, even if the interstate compact, on its face, says otherwise.

This is mostly because:

1. Federal law is supreme over state law, and

2. A later enacted law or treaty may overrule any previously adopted law or treaty (other than the U.S. Constitution) even if it says otherwise.

Answer 2

Here's an example:

Arizona v. California, 373 U.S. 546 (1963) wherein the Supreme Court held Congress acted within its realm of authority when it created a plan to manage and operate the Colorado River even though it had previously granted consent to the Colorado River Compact whose purpose was to assist in the management and operation of the body of water.

Also, your claim that "all compacts have to be authorized by Congress before coming into effect" isn't quite so.

the U.S. Supreme Court in Virginia v. Tennessee held that Congress must approve only two types of compacts:

• Those compacts that alter the balance of political power between the state and federal government; or
• Those compacts that intrude on a power reserved to Congress.