Wikipedia says: "An interstate compact is an agreement between two or more states of the United States of America. Article I, Section 10 of the United States Constitution provides that "no state shall enter into an agreement or compact with another state" without the consent of Congress. Frequently, these agreements create a new governmental agency which is responsible for administering or improving some shared resource such as a seaport or public transportation infrastructure."
I guess it could be argued that the Nevada/Utah agreement over the division of the Snake River Valley groundwaters must receive the blessing of Congress before it can be consumated. I believe that all the surface water interstate agreements (like the Republican River Compact between Colorado, Nebraska and Kansas) had to have Congressional approval. In these cases the sole issue was dividing up the supply of water.
I wish I knew more about the legal issues along these lines, but alas, I don't. Are there any legal types out there that would like to weigh in? For those opposed to the agreement, maybe a consultation with a legal firm with a strong Constitutional Law background would pay dividends.