The River That Was Divided Twice
The Colorado was divided between seven states in 1922 on the basis of flow records from an unusually wet decade. A century later it supplies forty million people, irrigates a large share of North America’s winter vegetables, and in most years does not reach the sea. The bathtub ring at Lake Mead is the visible edge of it.
Stand at Hoover Dam and look at the rock above the waterline of Lake Mead. There is a pale band running around the entire reservoir, tens of metres high, chalky where mineral deposits were left on stone that used to be submerged. Locals call it the bathtub ring, and it is the single clearest piece of physical evidence anywhere in the American West that the arithmetic underpinning the region does not add up.
The Colorado River supplies water to something in the order of forty million people across seven US states and two Mexican states, irrigates a very large share of the winter vegetables eaten in North America, and generates hydroelectricity for a substantial chunk of the Southwest. It has also, in most years of the last quarter century, been allocated more water on paper than actually flows down it.
A deal signed in a wet decade
The framework is the Colorado River Compact of 1922, negotiated at Bishop’s Lodge outside Santa Fe by representatives of the seven basin states with Herbert Hoover in the chair.
It divided the river into an upper basin and a lower basin and allocated 7.5 million acre-feet a year to each, with a further allocation to Mexico added by treaty in 1944. The total came to something close to 16.5 million acre-feet.
The problem is the flow figure they used. The gauge records available in 1922 came from an unusually wet run of years, and the negotiators assumed an average annual flow of around 16.4 million acre-feet. Tree-ring reconstructions since have put the long-term average considerably lower — and twenty-first-century flows lower still, with the period since 2000 the driest multi-decade stretch in the reconstructed record covering more than a millennium.
The gap between what is allocated and what arrives is known in water policy as the structural deficit, and it is not a drought that ends. Warming reduces snowpack, advances melt timing and increases evaporation and soil absorption, so a given amount of precipitation delivers less water to the river than it did a century ago. Researchers describe the condition as aridification rather than drought, which is a precise distinction: drought is a deviation, aridification is a new mean.
The river was divided on the basis of a wet decade, and every legal claim on it since has been built on that number. The water never showed up.
Two reservoirs holding the line
Lake Mead behind Hoover Dam and Lake Powell behind Glen Canyon Dam are the buffers that have absorbed the shortfall. Together they can store several years of the river’s flow, and for most of the twentieth century they were the reason the deficit was invisible.
By the early 2020s both had fallen to the lowest levels since they were filled, approaching elevations at which the dams could no longer generate power and, below that, the elevation at which water cannot be released downstream through the outlets at all — the condition known as dead pool. Emergency federal action and two unusually good snow years pulled them back from the immediate cliff. Nobody involved describes the reprieve as a solution.
The consequences are visible to any visitor. Glen Canyon’s falling water re-exposed side canyons that had been drowned since the 1960s, including parts of the landscape that the dam’s construction destroyed and that a generation of conservationists never forgave. Boat ramps at Mead now end in dry gravel hundreds of metres from the water. Lake Powell’s Hite marina has been unusable for years.
Who actually uses it
The public conversation is usually about lawns and swimming pools in Phoenix and Las Vegas, and that framing is misleading. Agriculture accounts for the substantial majority of Colorado River consumption — commonly estimated around three quarters or more — and within that, alfalfa and other cattle-feed crops are the largest single use.
That is not a moral indictment of farmers, who hold legal rights under a doctrine of prior appropriation that grants seniority by date of first use: first in time, first in right. Some of the most senior rights on the river are held by irrigation districts in the Imperial Valley of California, which means that in a genuine shortage, a farm established in the 1890s has a stronger legal claim than a city of two million founded later.
Cities have, meanwhile, done the impressive part of the story. Las Vegas returns essentially all indoor water use to Lake Mead through treatment and recycling, has banned ornamental turf, and has reduced total consumption substantially while its population grew. Tucson, Phoenix and Los Angeles have all cut per-capita use. Urban efficiency has bought time; it cannot close a gap of this size on its own.
And then there are the tribes. Thirty federally recognised tribes hold rights in the basin, some of them senior and some still unquantified, amounting collectively to a very large share of the river. Many were excluded from the 1922 negotiations entirely. Several communities with legal entitlements to substantial water — the Navajo Nation most prominently — have households without running water, because the entitlement exists on paper and the pipes do not.
The parks the river made, and the idea behind them
The other thing the Colorado produced is the American national park, which is one of the more consequential ideas the United States has exported.
Yellowstone in 1872 was the first area anywhere designated as a national park in the modern sense — land set aside by a national government for public benefit rather than for a monarch or a municipality. The model spread: Canada followed within two decades, then Australia, New Zealand, Argentina, South Africa, and eventually most of the world.
John Wesley Powell, the one-armed Civil War veteran who ran the Grand Canyon by boat in 1869 with no reliable map, later became the first serious voice arguing that the West could not be settled on the same terms as the East because there was not enough water. His 1878 report proposed organising the region by watershed rather than by survey grid, and limiting settlement to what the rivers could support. Congress ignored it comprehensively, and the twentieth century proceeded to test the proposition.
The dark side of the founding story is that these landscapes were not empty. The creation of parks and monuments across the West involved the removal or exclusion of Indigenous peoples from land they had used and managed for millennia, sometimes by treaty and sometimes by force. That history is now being addressed in park interpretation with more honesty than it was, and in a few places by co-management agreements: Bears Ears National Monument in Utah, established after a proposal from a coalition of five tribes, is managed under an arrangement that gives those nations a formal role.
Visiting the plateau means moving continuously between federal park land, Bureau of Land Management land, state land and sovereign tribal nations — Navajo, Hopi, Ute, Havasupai and others — each with their own rules, permits and fees. Assuming that a national park pass covers Monument Valley or Antelope Canyon is the single most common visitor error, and the answer is that both are Navajo, not federal.
What this looks like on the ground
For a traveller the Colorado is the reason the Southwest looks the way it does, and following it is a genuinely coherent way to see the region.
The Grand Canyon is the river’s work: 446 kilometres of it, cut through rock that records nearly two billion years, and best understood not from the rim but from inside — either on a raft trip through the whole canyon, which is a multi-week permit lottery, or on foot down to Phantom Ranch and back up, which is a serious undertaking that the park service spends every summer rescuing people from.
Upstream, Glen Canyon and the slot canyons around Page. Further up still, Canyonlands, where the Green and the Colorado meet at a confluence you can look down into from Island in the Sky, and Arches and Capitol Reef in the same sandstone. Downstream, the river runs to the Mexican border and then, in most years, stops — the delta at the Gulf of California, once a wetland of several thousand square kilometres, has been dry for decades, with limited restoration flows released under a binational agreement.
That is the fact to carry through the whole trip. The most legally contested river in the world does not reach the sea.
The deadline
The operating guidelines that govern how shortages are shared between the states expire in 2026, and the negotiation to replace them has been under way for years. The positions are entrenched: upper basin states argue their deliveries should reflect what nature provides; California defends senior rights; Arizona holds the junior position on the Central Arizona Project and would be cut first.
Everyone involved agrees that the compact allocates more water than exists. Nobody has an incentive to be the party that gives it up. Federal authority to impose a settlement exists in theory and has never been used, because doing so would end in the Supreme Court.
It is, in miniature, the general shape of every climate adaptation problem: a shared resource, a legal framework built for a wetter world, and a set of parties who each have a rational reason to hold their position until the water is physically gone. The bathtub ring at Mead is the visible edge of it, and it is worth going to look at.
Hiroshi covers rural Japan — the peninsulas, the fishing towns, the villages the shinkansen skips. His slow reporting on the Noto earthquake recovery has run in three languages.