In many river basins around the world, the instream flows in the mainstem rivers are often determined by the contribution of water to these mainstem rivers from tributaries. Notwithstanding the somewhat obvious hydrologic reality that the contribution of water from tributaries helps determine the instream flow of mainstem rivers, the law often treats mainstem rivers as if they were hydrologically distinct from tributaries. That is to say, legal rights of diversion of water on mainstem rivers are often determined without taking into proper account how diversion of water on upstream tributaries impacts the volume of water in mainstem rivers.

Perhaps the most glaring example of this disconnect between hydrology and the law is the case of the multistate legal regime (often referred to as the “Law of the River”) to allocate the Colorado River in the United States. Pursuant to the Law of the River, California was granted the right to divert 4.4 million acre-feet (MAF) of water from the Colorado River, while Arizona was granted the right to divert 2.8 MAF of water from the Colorado River.