The agreement has no army, little staff and almost no public profile. It governs roughly twenty percent of the planet's surface fresh water.

The Great Lakes–St. Lawrence River Basin Compact, ratified by eight American states and paired with an agreement with Ontario and Quebec, prohibits most diversions of water outside the basin's hydrological boundary. Its logic is simple: water that leaves the basin does not return, and the lakes are replenished by less than one percent of their volume each year.

Minnesota is a party by virtue of Lake Superior, and holds an unusual position — it is a basin state that also contains the headwaters of a continental river system flowing the other direction.

For two decades the compact was a subject for water lawyers. Drought has made it a subject for everyone else. As aquifers decline across the American Southwest and southern Plains, the political attractiveness of Great Lakes water rises, and the compact is the only thing standing between the lakes and a pipeline proposal.

Compact officials describe the diversion question as largely settled in law and entirely unsettled in politics. The legal structure is strong: any diversion outside the basin requires unanimous consent of all eight governors. The political structure is a different matter, because compacts are creatures of state legislatures and Congress consented to this one.

"The compact holds as long as the states want it to hold," a Canadian water official said. "That is true of every agreement between sovereigns. It is worth saying out loud occasionally."

The more immediate pressure is not diversion but straddling communities — municipalities that sit partly inside and partly outside the basin line, which may apply for exceptions under strict conditions including full return flow. Several such applications have been approved, each over objections, each establishing precedent in a framework designed to resist precedent.

Water quality has emerged as the parallel front. Nutrient runoff drives seasonal algal blooms in the shallower lakes, and while Superior remains comparatively cold and clean, warming water temperatures have extended the season in which blooms are possible.

Minnesota's contribution to the problem is smaller than that of states farther south, which has not spared it from the argument. Basin-wide nutrient reduction targets require action from every jurisdiction, and the ones with the least runoff have the least political incentive to act.

What makes the compact durable, its defenders argue, is that it was negotiated before the crisis rather than during one. It was signed in a period when no one was proposing a pipeline, which is exactly why agreement was possible.

"Every good water agreement in history was signed during a wet year," the Canadian official said. "That is not a coincidence. That is the whole technique."