How Dry was 2000-2018 on the Colorado Compared to “Normal”?

By Eric Kuhn

The Colorado River’s natural flows are shrinking by 9% per degree C (1.8 F) of warming as climate change continues to sap the river’s flow, according to an important new study by Chris Milly and Krista Dunne of the US Geological Survey. Milly and Dunne also conclude that increasing precipitation is unlikely to offset this temperature induced drying. The study adds important additional evidence that not only will climate change reduce the river’s flows in the future, but that it is already happening – the already over-tapped Colorado River is facing a future with even less water.

tree ring reconstruction of the Colorado River’s flow, from Woodhouse, Connie A., Stephen T. Gray, and David M. Meko. “Updated streamflow reconstructions for the Upper Colorado River basin.” Water Resources Research 42.5 (2006).

My one caution is that like almost all other similar studies, for baseline purposes, Milly and Dunne use the Colorado River Natural Flow Data Base (NFDB) for the annual natural flows at Lee Ferry.  The Natural Flow Data Base is maintained by Bureau of Reclamation scientists based at CADSWES in Boulder, Colorado. The data base shows monthly and annual natural flows at critical points in the basin including Lees Ferry. It is updated approximately annually when the most recent year for which data are available is added.  The most recent version was released in February 2020 and covers water years 1906-1918.  The version used by Milly and Dunne was released in March 2019 covering 1906-2017.

While the Natural Flow Data Base provided an important source of baseline information for Milly and Dunne, as well as other recent studies of the impact of climate change on the Colorado River, it is important to be aware of its shortcomings.

Uncertainty about early data

The farther we go back in time, the less confidence we should have in the data.  Natural flows are gauged flows adjusted for upstream human hydrologic modifications. Before 1930 there were relatively few gauging stations in the entire basin and before June 1921 there was no gauge at the critical Lees Ferry location (Water Year 1922 is the first full year of measured flows at Lees Ferry). The major sources of consumptive use above Lees Ferry are irrigated agriculture (well over 90% of the use before the major dams and export projects were built beginning in the 1950s). Irrigation uses are the most critical variables used to calculate the natural flows reported in the NFDB, but before the late 1940s the data available on acres under irrigation and cropping types were very sparse. The major source of data was the federal irrigation census conducted every five to ten years. Beginning in the late 1940s, project planning efforts by the Bureau of Reclamation led to much more and better data on irrigation use.

Start date

Although the NFDB begins with Water Year 1906, both the Bureau of Reclamation and USGS published data sets with estimated annual natural flows at Lees Ferry starting well before 1906. In Science Be Dammed John Fleck and I make the case that 1906 was likely picked in the 1960s by Commissioner Floyd Dominy to give the Central Arizona Project the best water yield (read the book). It is a decision we live with today. Before the 1960s, Reclamation studies routinely used natural flows back into the 1890s. There are annual natural flow estimates for Lee Ferry of comparable quality to the pre-Lees Ferry gauge estimates that go back to about 1878.  Why is this important? The early 20th Century pluvial began in 1905/06 and lasted through about 1930. In the three decades before 1905 conditions in the Colorado River basin were much drier, similar to the Colorado River Basin in the 1930s to 50s (thus Dominy’s marketing decision). The choice one makes about which period of record to use makes a difference in how the river’s estimated mean annual flow:

  • 1906-2018: 14.8 million acre-feet per year (MAF/yr).
  • 1878-2018: 14.4 MAF/yr.

Based on tree-ring based reconstructions we now understand that the 1906-30 pluvial was not only wet, it was extraordinarily wet. In an observation first made by Charles Stockton and Gordon Jacoby in their seminal 1976 paper reconstructing Lee Ferry Flows back over 400 years, they conclude that the periods of record used by the Bureau of Reclamation that include the 1906-30 pluvial will be skewed to the wet-side. Based on more recent reconstructions, we now believe that 1906-30 was one of the two or three wettest 25-year periods in the last 1400 years (perhaps even the wettest). The long-term average natural flow at Lees Ferry from the most recent reconstructions ( 2018) is about 14.3 MAF/yr.

Implications of the data’s shortcomings

What are the implications of the cautions I’ve listed?  For starters, when comparing the recent 2000-2018 “drought” period with periods of record that begin with or include major portions of the 1906-30 pluvial, the magnitude of the current “drought” is often overstated.

Based on the NFDB, the mean annual natural flow at Lees Ferry for 2000-2018 was 12.47 MAF/yr, 15.5% less than the 1906-2018 average of 14.76 MAF/yr (“normal”).  However, using a different baseline such as the post-pluvial period of 1931 -2018, that same drought period is only 10% less than “normal”. Given the uncertainties in the natural flow data base before 1930, the use 1931-2018 is a reasonable decision (and one that is gaining favor with some of the basin’s water agencies).

For research purposes, study authors might want to give more consideration to the uncertainties in the NFDB. They should consider using a periods of record starting in 1922, after there was an actual gauge at Lees Ferry, or 1931, the post pluvial period and the year after the Boulder Canyon Project Act funded additional gauging stations, or even 1948, the year soil scientists, Blaney and Criddle, were tasked with improving our understanding of irrigation consumptive uses in the Upper Basin. If possible, study authors may even want to compare their results with the long-term paleo record mean of 14.3 MAF/yr. All are reasonable approaches that will yield slightly different results.  As with Milly and Dunne, for many studies, the period of record used may be driven by other data limitations.

The bottom line is that there are a range of different answers to the question of what is the normal flow of the Colorado River at Lees Ferry. Milly and Dunne, along with other researchers have used the Natural Flow Data Base to help us understand the impact of climate change on the Colorado River, have made critical contributions. But as we move forward with our policy responses, we need to be open about the uncertainties inherent in the data we are using. An effort to study the sensitivity of climate change analyses to the uncertainties in the NFDB would be a valuable addition to our understanding.


  1. Eric is right to analyze the assumptions. Especially when decisions were motivated by financial gain.

    Climate change predictions world wide have underestimated the rate and amount of change. Scientists wanted to be on the conservative side. However, the force of the feedback loops increases with higher temperatures. The effects compound in the aggregate.

    Plan for the worst and hope for the better? Where is the harm in assuming less flow in the river; and bigger decreases going forward? A huge amount of changes need to be made; not all of which will happen voluntarily.

    With all of the global warming problems: try to prepare now or get hammered as the problems become overwhelming. Also, addressing fossil fuel use in the southwest could possibly delay some of the damage.

  2. Problem is already overwhelming unless corona virus provides some respite from demand and climate change, which is unlikely, and the power of the multi-veto and the $ will make problems worse. The four horsemen of the apocalypse have been joined by a fifth or sixth. Lack of water and Lack of capital. For us I told you so’s, it grimly laughable as we watch newbies wrastle to prove Le Chatlier’s principle wrong in the struggle to prove that water is not a zero sum game. Darn it: You must be able to put Humpty Dumpty Together Again.” I hate to be such a pessimist. But when everyone realizes you can’t you’ll be out of a job and Hope does not sustain body and soul and we will return to the days of Sumer.


    William J. Turner

    I seem to recall that many years ago I read a story in which a good-humored idiot had offended the Empress by taking liberties with her. For this act and to serve as a cautionary example to other subjects, by Imperial decree he was driven through the streets of all the towns in the Empire. Each citizen was required to watch his passage on his way to torture and execution. Neither the citizens nor the idiot had knowledge of the ghastly tortures in store for him, nor would they until the torture began. Believing that all those who lined the streets were his admiring friends, he beamed and bestowed on them his happiest smiles and jokes.

    Now consider. On the 27th day of June, in the year of our Lord, 1988, we found in the New York Times some news that was fit to print: “With evidence mounting that the earth is becoming a hotter place to live, the United States and many other nations are beginning to plan for climatic change caused by what environmentalists call the greenhouse effect.” In other publications we have been told that as the polar ice caps melt the sea level is expected to rise one foot in the next 40 years. Changes in the climate are expected to lead to still-unpredictable but drastic alterations in rainfall, wind patterns, and oceanic currents. The cause of this change is found to lie in the increasing carbon dioxide (CO2) concentration in the atmosphere. This, in turn, is due to burning of our forests and fossil fuels, i.e., gas, oil, and coal.

    Perhaps it is best at this point to call to mind that we cannot legislate away the three laws of thermodynamics. We cannot escape from two most profound further principles of universal and widespread application derived from these laws. The first is the Principle of le Chatelier, also known as The Principle of Least Effort: When a force is applied to any system in equilibrium it at once induces a countervailing action which limits the effect of the first. The common example is known through the pumping up of a bicycle tire; the effectiveness of the compression by the pump is lessened by heating up of the air. This principle, so easily exemplified, leads in extension to inexorable consequences. One of these is so simply statable that it does not betray its fearful subtlety and imminence: You can not make everyone wealthy, but you can make everyone poor. Further, there is an inescapable relationship between the numbers of those in each level of wealth and the total wealth of a population: one at the top; millions at the bottom. It is not wholly funny nor nonsensical to say that 2+2=4 -TdS.

    The second principle, generally known as The Catastrophe Theory, has been developed by many mathematicians since its stormy introduction. This deals with action which, up to a point, or a cusp, does not cause an irremediable consequence. A common example is tension on a string, a rubber band, an egg shell, or a bridge girder: up to the cusp they stretch or bend; beyond that they break. Of course, the first principle begins to act at the instant of the first stretch or bend. This principle, too, leads, in its formal development, to conclusions which can also be so badly stated as to hide their fearful implications: You can’t get home again; if you try the quick fix you blow it; if you do it the slow way you end up somewhere else, but maybe safe. It is easiest to make everyone poor.

    I have not enough information to estimate how far along we are toward a cusp in global heating. Nor do I have secure information as to whether or not we are still on the self-correcting, homeostatic, healing side of the ozone hole in the polar atmosphere. (No, Virginia, it is not only in the Antarctic!) But in mind of the fact that we are still producing enormous quantities of fluor- and polychlorinated hydrocarbons, the accumulation of which in the troposphere is a long, slow process, I am frightened enough to imagine that we have passed the point of no return. I do not want to think of the short-term (100 years) consequences of our abuse of our planet, though I must. I do not want to project the long term (10 and more millennia) effects either, any more than I want to ingest the organic mercury compounds derived from the tons of mercury “lost” at Hanford and elsewhere.

    Well now that we have gotten off to a good start in forecasting a black and miserable future, we find much more cause to view with alarm.

    Item. In nearly every issue of National Geographic, Scientific American, Science, and other learned journals accessible to any one who reads, there are depictions of the laying waste of the forests of the world. Throughout every land people cut timber for fuel, for building, for land on which to live and cultivate. Population explosions lead to ever-increasing demand for wood products, including paper for 16 copies of everything. In the US the vast forests which once covered the land have given way to farms, cities, and paved roads. The US is preparing to denude Alaska, unless forest fires do it first. In Brazil hundreds of thousands of acres of forests, with all their forms of life, are being burned every year, to provide for farmland. It is well known that such farmland has poor soil which shall turn to useless Laterite in a few years. The soil cannot be restored for thousands of years. Such deforestation in India has been a significant factor in the excessively severe floods of Bangladesh this year. These floods have left 21 million people homeless in 1988. Five years hence they may well kill that many.

    It is authoritatively estimated that by the year 2050 (60 years from now, only 60!) all of the rain forests of the world shall have disappeared and been replaced by deserts containing no mammalian life. According to the World Population News Service (Popline, June, 1988) “Some 65 million hectares of farming land in sub-Sahara Africa have been turned to desert over the last 50 years and the process is accelerating.” In the same issue of Popline Dr. Safir Sadik wrote: “The ‘scissors effect’ of poverty and increasing population is slicing away at the ability to sustain human life.” In the same report she added: “The environmental costs of hazardous industrial technologies are mounting and there are already fears that the damage may be irreversible.” Read that to mean: “Our very population level is such that the damage may be irreversible.” This is not limited to Africa and India; it is world wide. We have a desert in Maine. Although the drought of 1988 is expected in the 16-year cycle long known in the US, possibly this one is exaggerated by our climatic warming. The fires now raging in the US, Canada, and Alaska in this summer of 1988 seem to bode still further disaster. We remove the trees and other plants from the shores of lakes and streams to build waterfront property, thus destroying Nature’s water-cleansing resources. The tree/shrub belts planted in the late 1930’s, to serve as windbreaks and soil preservers, have been torn out to enlarge the cultivated fields in the mid-West. We do not plant trees or other cover to protect our watersheds. Were the CO2 increase to occur without deforestation we might hope that the Principle of Least Effort would come into effect. With warmer climate trees would grow faster and remove the CO2 to a new balance. Without the trees. . . catastrophe.

    In deforestation we prevent replenishment of our underground water tables, while simultaneously washing away the soil. We simply must recognize that we are creating desert world in which the balance of Nature is being altered too fast and too much. Water runs off after rain; the land heats up, and the rains cease to fall. The deserted fields, to hastening ills fall prey. The desert yields no grain; cattle do not survive; in the end, human do not survive there either.

    Item. Suppose that mid-East war stops for good and there may be no more destroying of oil fields. Suppose that many more offshore fields are opened up. At the 1988 rate of use, with no more wars and allowing that we may find ways of extracting all the oil there is, maybe the world’s supply will last another 50 years of plenty, and another 50 or more of exhaustion. All the while carbon dioxide, as well as oxides of nitrogen and sulfur, pour into the atmosphere. If there were forests they might grow faster in an “enriched” atmosphere even with acid rain. But, with no more forests?? Also, what if third world countries become “developed” and need more oil? For that matter, the powers that be are certain to demand more oil for expanding economies. I do not think it necessary to be quantitative about it, nor to detail the exponential rate of increasing use of oil, to point out that we, like our happy idiot, have no sense of immediacy of the Empress’ torture.

    Oh, also, the burning of this oil and its chemical transformations with fluorine and chlorine lead not only to the ozone hole, but also to extensive pollution of all the waters and much of the soil of the earth. As TIME had it (Aug. 1, 1988): “Threatened by rising pollution, the oceans are sending out an SOS.” The New York Times accounts tell us about the extensive pollution of the waters surrounding Europe and North Africa. No whales have been seen off Montauk Point this summer. Seals off Norway and off California are dying by the thousands from viral infections (canine distemper!) and from poisoning by industrial wastes. Hospital waste is a trifle in this state of affairs.

    The known reserves of gas far exceed those of oil. Europe has a good deal, but the USSR has so much it has not been possible to make more than a rough guess. Now that Chernobyl has brought about a certain degree of caution in the use of nuclear energy, that gas is rapidly being brought out. Still, it is not infinite, and in the meanwhile the burning produces more CO2 as well as more oxides of nitrogen.

    Consider further sources of atmospheric pollution. There is coal and lignite already known in the US of sufficient quantity to last another 300 years. And still unknown huge fields elsewhere; maybe 600 years? What is in store for our grandchildren? Suppose these fuels do last 600 years. It is only 500 years since the discovery of America. Not a very long time in human history and insignificant in the history of life on this planet. Do we have the social and genetic adaptability to live in the world ahead?

    We can not leave the matter of energy and pollution without considering the wonders and risks of nuclear energy. While some nations, particularly France and Japan, have been able to reduce significantly their dependence on oil by gas and by nuclear fission plants the horrors to come make Three Mile Island and Chernobyl kindergarten exercises. Every scientist outside of the Nuclear Regulatory Commission knows that these plants have a limited life: 30 years? 40 years? 50, at the most. Then they must be dismantled and . . . . . and what? No one really knows. Every idea so far advanced turns out to be either impractical or impossible. The idea of encasing the “short-life” (up to 30,000 years) isotopes in stainless steel drums makes me choke with tears. This is partly because the Canadians are already doing it, and are placing such drums in deep wells. Stainless steel, indeed! 30,000 years? Or even 300! Canisters supposed to encase radioactive waste have already burst within ten years of preliminary storage. The further idea of placing the “long life” isotopes in glassified blocks in deep caverns in tectonically stable regions has been furiously criticized by geologists. The huge mines in the US, supposedly dry and safe for long-term storage of nuclear waste, are already found to have seeping water and are probably due for abandonment.

    The problems alluded to above are greatly magnified by the tendency of administrators to nuclear plants to ignore their responsibilities. In October 1988 we are informed of some 10-20 “failures” yearly for the past 20 years at the Savannah River reactor in South Carolina. Surely this is not the only such reactor with “failures.” Word is that at Hanford the whole community is indifferent to such possibilities. We have workers on drugs or alcohol, Union rules, administrators careful of their jobs and retirement benefits . . . and so on. Why should we expect those at a nuclear plant to be a different species than those elsewhere?

    See The Bulletin of the Atomic Scientists over the past 30 years. This Bulletin is open to anyone who wants to subscribe to it or has access to a library. Oh, well, in another hundred years maybe there will not be anything alive to notice: the Empress’s edict shall long ago have been carried out. Meanwhile we are faced with the problem of disposing of enormous amounts of highly radioactive waste. WHO CAN BELL THESE CATS?

    Humans have always moved to destroy the next habitat, either in the adjoining land, or by emigration. Now there is really no place for the emigration of burgeoning hundreds of millions. They are poor. They starve by the millions. The human reproduction potential leads to doubling of the population every few decades. There are many cultures in which it has been necessary to have 12 children for 2 to survive. They starve in Indochina or India, or Central America. They come to the United States, or to Europe. They glory on their fecundity. Our population is now officially stated to be 245,000,000. The fecundity upon which their survival depended is now a frightful threat. Here, all can survive. And all, they, we, I, call ever for more of the amenities: freedom from want, from hunger, from fear. People’s uprisings occur, to the dismay of those of us who have nice towns, roads, supermarkets, automobiles, even $30,000,000 yachts, supercolliders, all sorts of exotic weapons that do not work.

    Item. Hey! I almost forgot. To support our “civilization” not only do we use fuels and wood. We also use minerals. There is plenty of silicon and probably water, sodium, and calcium. But usable deposits of iron are running low. If Asia, the USSR, Africa, and South America ever get highly industrialized the iron won’t go around, nor the cobalt, zinc, tin, aluminum, sulfur, or helium, to mention but a few which we find essential to our level of desired comfort. Maybe before we run out of minerals we shall run out of fuel, but perhaps we can make do with coca leaves. But, Doctor Pangallos, there will be no coca leaves either, what from acid rain and spewing of toxic clouds from industrial plants.

    As I come to the end of this aspect of human future, it is fitting to try to understand the source of this future. To put it briefly: there are too many of us, and we each want more and more. OK. We all know that. We know that the population of the planet is now more than 5 billion humans and counting, and that we are killing off all other species of plant and animal. We are all set to kill ourselves off. Indeed, Man is the only known species which delights in torture and killing: atrocity for its own sake.

    As I read the evidence, about 500,000-200,000 years ago an oocyte in Mother Eve underwent a mutation in its mitochondrial DNA. The Homo sapiens which arose from this mutation evidently could not endure the existence of others. It killed its neighbors; it had poor opinion of its kin; moved into other territories and in so doing killed off the Neanderthal and Cro-Magnon Man, and Peking Man among . . . . indeed all others, spread world-wide . . There is a beautiful map of our migrations in the October, 1988 issue of National Geographic. So here we are: unable to live with ourselves, unable to live without ourselves, constantly at war within the individual, between sibs, neighbors, nations, and all of Nature. Unless we can find other aspects of our genetic inheritance to overcome the paranoid Mother Eve in us we are doomed to be an experiment of Mother Nature (the only aim of which seems not so much to glorify God as to synthesize more DNA), coming to bright blossom and promise, only to fail. The meek are not destined to inherit the earth, but neither are the rest of us.

    Unless we can control population, down to the level at which we use resources only to the extent that we exist in homeostasis, in equilibrium, with our environment, we shall inevitably die off by operation of forces beyond our control: as certainly as the principles of le Chatelier and of Catastrophe theory are universal and absolute. War is inevitable (47 going on as of October 1, 1988, give or take as many more massive killings), and sooner or later a desperate use of atom bombs shall occur. The growing starvation rates in many countries are appalling, occurring to a dreadful state even in the US (despite President Reagan’s inability to find any in this Nation). Financial failures, with resultant inability to maintain public health measures, are sure to bring pestilence to nations now relatively secure. Now, also, the Four Horsemen are joined by a fifth: mutations resulting from chemical and nuclear actions. What sort of being is to take our place on this planet?

    Maybe I did not read the story of the idiot. Maybe this is all a dream, a tale told by an idiot, for it seems in my mind’s eye that I am the idiot, and all those who watch my progress look exactly like me.

    October 6, 1988

  4. it is amazing to me how some people will disregard tree ring or air bubble data from glaciers when it is right there…

Comments are closed.