[Note that this article is a transcript of the video embedded above.]

In June of 2022, the level in Lake Mead, the largest water reservoir in the United States formed by the Hoover Dam, reached yet another all-time low of 175 feet or 53 meters below full, a level that hasn’t been seen since the lake was first filled in the 1930s. Rusted debris, sunken boats, and even human remains have surfaced from beneath the receding water level. And Lake Mead doesn’t stand alone. In fact, it’s just a drop in the bucket. Many of the largest water reservoirs in the western United States are at critically low storage with the summer of 2022 only just getting started. Lake Powell upstream of Lake Mead on the Colorado River is at its lowest level on record. Lake Oroville (of the enormous spillway failure fame) and Lake Shasta, two of California’s largest reservoirs, are at critical levels. The combined reservoirs in Utah are below 50% full. Even many of the westernmost reservoirs here in Texas are very low going into summer.

People use water at more or less a constant rate and yet, mother nature supplies it in unpredictable sloshes of rain or snow that can change with the seasons and often have considerable dry periods between them. If the sloshes get too far apart, we call it a drought. And at least one study has estimated that the past two decades have been the driest period in more than a thousand years for the southwestern United States, leading to a so-called “mega-drought.” Dams and reservoirs are one solution to this tremendous variability in natural water supply. But what happens when they stop filling up or (in the case of one lake in Oklahoma), what happens when they never fill up in the first place? I’m Grady, and this is Practical Engineering. On today’s episode we’re talking about water availability and water supply storage reservoirs.

The absolute necessity of water demands that city planners always assume the worst case scenario. If you have a dry year (or even a dry day), you can’t just hunker down until the rainy weather comes back. So the biggest question when developing a new supply of water is the firm yield. That’s the maximum amount of water the source will supply during the worst possible drought. Here’s an example to make this clearer:

Imagine you’re the director of public works for a new town. To keep your residents hydrated and clean, you build a pumping station on a nearby river to collect that water and send it to a treatment plant where it can be purified and distributed. This river doesn’t flow at a constant rate. There’s lots of flow during the spring as mountain snowpack melts and runs off, but the flow declines over the course of the summer once that snow has melted and rain showers are more spread out. In really dry years, when the snowpack is thin, the flow in the river nearly dries up completely. In other words, the river has no firm yield. It’s not a dependable supply of water in any volume. Of course, there is water to be used most of the time, but most of the time isn’t enough for this basic human need. So what do you do? One option is to store some of that excess water so that it can keep the pumps running and the taps flowing during the dry times. But, the amount of storage matters.

A clearwell at a water treatment plant or an elevated water tower usually holds roughly one day’s worth of supply. Those types of tanks are meant to smooth out variability in demands over the course of a day (and I have a video on that topic), but they can’t do much for the reliability of a water source. If the river dries up for more than one day at a time, a water tower won’t do much good. For that, you need to increase your storage capacity by an order of magnitude (or two). That’s why we build dams to create reservoirs that, in some cases, hold trillions of gallons or tens of trillions of liters at a time, incredible (almost unimaginable) volumes. You could never build a tank to hold so much liquid, but creating an impoundment across a river valley allows the water to fill the landscape like a bathtub. Dams take advantage of mother nature’s topography to form simple yet monumental water storage facilities.

Let’s put a small reservoir on your city’s river and see how that changes the reliability of your supply. If the reservoir is small, it stays full for most of the year. Any water that isn’t stored simply flows downstream as if the reservoir wasn’t even there. But, during the summer, as flows in the river start to decrease, the reservoir can supplement the supply by making releases. It’s still possible that in those dry years, you won’t have a lot of water stored for the summer, but you’ll still have more than zero, meaning your supply has a firm yield, a safe amount of water you can promise to deliver even under the worst conditions, roughly equal to the average flow rate over the course of a dry year.

Now let’s imagine you build a bigger dam to increase the size of your reservoir so it can hold more than just a season’s worth of supply. Instead of simply making up a deficit during the driest few months, now you can make up the deficit of one or more dry years. The firm yield of your water source goes up even further, approaching the long-term average of river flows, and completely eliminating the idea of a drought by converting all those inconsistent sloshes of rain and snow into a perfectly constant supply. Beyond this, any increase in reservoir capacity doesn’t contribute to yield. After all, a reservoir doesn’t create water, it just stores what’s already there.

Of course, dams do more than merely store water for cities that need a firm supply for their citizens. They also store water for agriculture and hydropower that have more flexibility in their demand. Reservoirs serve as a destination for recreation, driving massive tourism economies. Some reservoirs are built simply to provide cooling water for power plants. And, many dams are constructed larger than needed for just water conservation so they can also absorb a large flood event (even when the reservoir is full). Every reservoir has operating guidelines that clarify when and where water can be withdrawn or released and under what conditions and no two are the same. But, I’m explaining all this to clarify one salient point: an empty reservoir isn’t necessarily a bad thing.

Dams are expensive to build. They tie up huge amounts of public resources. They are risky structures that must be vigilantly monitored, maintained, and rehabilitated. And in many cases, they have significant impacts on the natural environment. Put simply, we don’t build dams bigger than what’s needed. Empty reservoirs might create a negative public perception. Dried up lake beds are ugly, and the “bathtub ring” around Lake Mead is a stark reminder of water scarcity in the American Southwest. But, not using the entire storage volume available can be considered a lack of good stewardship of the dam, and that means reservoirs should be empty sometimes. Why build it so big if you’re not going to use the stored water during periods of drought? Storage is the whole point of the thing… except there’s one more thing to discuss:

Engineers and planners don’t actually know what the worst case scenario drought will be over the lifetime of a reservoir. In an ideal world, we could look at thousands of years of historical streamflow records to get a sense of how long droughts can last for a particular waterbody. And in fact, some rivers do have stream gages that have been diligently collecting data for more than a century, but most don’t. So, when assessing the yield of a new water supply reservoir, planners have to make a lot of assumptions and use indirect sources of information. But even if we could look at a long-term historical record as the basis of design, there’s another problem. There’s no rule that says the future climate on earth will look anything like the past one, and indeed we have reason to believe that the long-term average streamflows in many areas of the world - along with many other direct measures of climate - are changing. In that case, it makes sense to worry that reservoirs are going dry. Like I said, reservoirs don’t create water, so if the total amount delivered to the watershed through precipitation is decreasing over time, so will a reservoirs firm yield

That brings me to the question of the whole video: what happens when a reservoir runs out of water? It’s a pretty complicated question, not only because water suppliers and distributors are relatively independent of each other and decentralized (capable of making very different decisions in the face of scarcity), but also because the effects happen over a long period of time. Most utilities maintain long-term plans that look far into the future for both supply and demand, allowing them to develop new supplies or implement conservation measures well before the situation becomes an emergency for their customers. Barring major failures in government or public administration, you’re unlikely to turn on your tap someday and not have flowing water. In reality, water availability is mostly an economic issue. We don’t so much run out as we just use more expensive ways to get it. Utilities spend more money on infrastructure like pipelines that bring in water from places with greater abundance, wells that can take advantage of groundwater resources, or even desalination plants that can convert brackish sources or even seawater into a freshwater source. Alternatively, utilities might invest in advertising and various conservation efforts to convince their customers to use less. Either way, those costs get passed down to the ratepayers and beyond.

For some, like those in cities, the higher water prices might be worth the cost to live in a climate that would otherwise be inhospitable. For others, especially farmers, the increased cost of water might offset their margins, forcing them to let fields fallow temporarily or for good. So, while drying reservoirs might not constitute an emergency for most individuals, the impacts trickle down to everyone through increased rates, increased costs of food, and a whole host of other implications. That’s why many consider what’s happening in the American southwest to be a quote-unquote “slow moving trainwreck.”

In 2019, all the states that use water from the Colorado River signed a drought contingency plan that involves curtailing use, starting in Arizona and Nevada. Those curtailments will force farmers to tap into groundwater supplies which are both expensive and limited. Eventually, irrigated farming in Arizona and Nevada may become a thing of the past. There’s no question that the climate is changing in the American Southwest, as years continue to be hotter and drier than any time in recorded history. It can be hard to connect cause and effect for such widespread and dramatic shifts in long-term weather patterns, but I have one example of an empty reservoir where there’s no question about why it’s dry.
In 1978, the US Army Corps of Engineers completed Optima Lake Dam across the Beaver River in Oklahoma. The dam is an earth embankment 120 feet (or 37 meters) high and over 3 miles or 5 kilometers long. The Beaver River in Oklahoma had historically averaged around 30 cubic feet or nearly a cubic meter per second of flow and the river even had some major floods, sending huge volumes of water downstream. However, during construction of the dam, it became clear that things were rapidly changing. It turns out that most of the flows in the Beaver River were from springs, areas where groundwater seeps up to the surface. Over the 1960s and 70s, pumping of groundwater for cities and agriculture reduced the level of the aquifer in this area, slashing streamflow in the Beaver River as it did. The result was that when construction was finished on this massive earthen dam, the reservoir never filled up. Now Optima Lake Dam sits mostly high and dry in the Oklahoma Panhandle, never having reached more than 5 percent full, as a monument to bad assumptions about the climate and a lesson to engineers, water planners, and everyone about the challenges we face in a drier future.