What Happens When a Reservoir Goes Dry?

Summary

In 2022, Lake Mead, the largest US reservoir, hit an all-time low, reflecting a broader crisis in western US water storage. The video explores the critical role of reservoirs in managing water supply, emphasizing 'firm yield'—the reliable water supply during droughts. It discusses the importance of dam and reservoir infrastructure, the economic implications of water scarcity, and the challenges of planning for future water needs in the face of climate change. The script also highlights the case of Optima Lake Dam, a symbol of the complexities in water resource management.

Takeaways

• 🌊 In June 2022, Lake Mead, the largest water reservoir in the U.S., reached an all-time low of 175 feet below full, revealing rusted debris and sunken boats.
• 🔍 Many major western U.S. reservoirs, including Lake Powell, Lake Oroville, and Lake Shasta, are critically low, indicating a widespread water crisis.
• 🌧️ The southwestern U.S. has experienced a 'mega-drought,' making water supply variability and storage a critical issue.
• 🏗️ Dams and reservoirs are crucial for managing water supply variability, but they face challenges when they fail to fill up or never fill up at all.
• 💧 The concept of 'firm yield' is central to water supply planning, representing the maximum amount of water a source can supply during the worst drought.
• 🌨️ Natural water supply is inconsistent, with significant dry periods, necessitating the storage of excess water during wet periods for use during droughts.
• 🛠️ Reservoirs can increase a water source's firm yield by storing enough water to cover deficits over multiple dry years, effectively eliminating drought impacts.
• 🌾 Beyond urban water supply, reservoirs serve multiple purposes including agriculture, hydropower, recreation, and providing cooling water for power plants.
• 💹 Water scarcity is primarily an economic issue, with utilities investing in infrastructure or conservation to secure water supplies, costs that are passed on to consumers.
• 🌿 The American Southwest's changing climate, marked by hotter and drier conditions, is leading to long-term impacts on water availability and the need for adaptive strategies.

Q & A

What was the all-time low level of Lake Mead in June 2022?

In June 2022, Lake Mead reached an all-time low of 175 feet or 53 meters below full.

Why are many water reservoirs in the western United States at critically low storage?

Many water reservoirs in the western United States are at critically low storage due to a combination of factors including the summer season, a long-term trend of decreasing water levels, and a 'mega-drought' that has been the driest period in over a thousand years for the southwestern United States.

What is the firm yield in the context of water supply?

The firm yield refers to the maximum amount of water a source will supply during the worst possible drought, ensuring a dependable supply even in dry years.

How do dams and reservoirs help manage the variability in natural water supply?

Dams and reservoirs help manage variability in natural water supply by storing excess water during periods of high flow and releasing it during dry periods, thus providing a more consistent supply.

Why is it important for city planners to consider the firm yield when developing a new water supply?

City planners must consider the firm yield to ensure a reliable and consistent water supply for their residents, even during the worst drought conditions.

What is the purpose of building larger reservoirs beyond a season's worth of supply?

Building larger reservoirs allows for the storage of water over multiple dry years, increasing the firm yield and reducing the impact of droughts by providing a more constant supply.

How do reservoirs contribute to agriculture and hydropower?

Reservoirs store water for agriculture and hydropower, which have more flexible demand, and can also serve as a destination for recreation and tourism.

Why might an empty reservoir not necessarily be a bad thing?

An empty reservoir might not be a bad thing because it can indicate good stewardship of the dam, ensuring that the stored water is used during periods of drought, and also because dams are not built larger than necessary.

What are the economic implications of drying reservoirs?

Drying reservoirs can lead to increased water prices, higher costs of food, and other economic impacts as utilities invest in more expensive water sources or conservation measures.

Why did Optima Lake Dam in Oklahoma never fill up after its construction?

Optima Lake Dam never filled up because during its construction, groundwater pumping for cities and agriculture reduced the aquifer level, which in turn slashed the streamflow in the Beaver River, leading to insufficient water to fill the reservoir.

How does Brilliant, the sponsor of the video, relate to the topic of water supply and engineering?

Brilliant is a learning platform for STEM subjects, including civil engineering, which is relevant to the design and management of water supply systems like dams and reservoirs discussed in the video.

Outlines

00:00

🌊 Drought and the Struggle of Water Reservoirs

The paragraph discusses the critical situation of water reservoirs in the western United States, particularly Lake Mead, which reached an all-time low in June 2022. The script highlights the broader issue of water scarcity, mentioning other reservoirs like Lake Powell, Lake Oroville, and Lake Shasta, which are also at dangerously low levels. The narrative delves into the concept of 'firm yield,' which is the reliable water supply a source can provide even during the worst droughts. The importance of dams and reservoirs in managing water supply variability is underscored, along with the challenges posed by climate change and the so-called 'mega-drought' in the southwestern United States.

05:06

💧 The Function and Management of Reservoirs

This section explains the purpose of reservoirs beyond just water storage, including their roles in agriculture, hydropower, recreation, and flood control. It emphasizes that reservoirs should sometimes be empty, reflecting prudent water resource management rather than failure. The paragraph also addresses the economic and practical challenges of dam construction and maintenance, as well as the unpredictability of water supply due to climate change. The video script points out that water scarcity is primarily an economic issue, with utilities employing various strategies to ensure supply, such as investing in infrastructure or promoting conservation. The impacts of these strategies are felt by consumers through increased water rates and food costs.

10:08

🏞️ The Legacy of Misjudgment: Optima Lake Dam

The final paragraph shares a cautionary tale about the Optima Lake Dam in Oklahoma, which was completed in 1978 but never filled due to over-pumping of groundwater that reduced the natural flow of the Beaver River. This example illustrates the complexities and uncertainties in water resource planning and management. The script suggests that while droughts are often seen as simple reservoir level indicators, they involve intricate interplays of hydrology, geology, meteorology, and civil engineering. The paragraph concludes by advocating for a multidisciplinary approach to solving water supply challenges and promoting the sponsor, Brilliant, as a platform for learning STEM subjects that are crucial for addressing such complex issues.

Keywords

💡Lake Mead

Lake Mead is the largest water reservoir in the United States, formed by the Hoover Dam. It is a critical water source for the Southwest, but as the script describes, it reached an all-time low in June 2022, highlighting the severity of water scarcity issues. The lake's low levels have exposed rusted debris and even human remains, emphasizing the impact of drought on water storage.

💡Drought

Drought refers to a prolonged period of abnormally low rainfall, leading to a shortage of water. In the context of the video, it is discussed as a significant factor contributing to the critically low storage levels of many water reservoirs in the western United States, including Lake Mead. The video mentions a 'mega-drought,' indicating an extended and severe drought.

💡Reservoir

A reservoir is an artificial lake or storage space for water, often created by building a dam across a river. Reservoirs are crucial for managing water supply variability, as they store water during periods of high flow and release it during dry spells. The video explains how reservoirs like Lake Mead and Lake Powell are at record low levels, affecting water availability.

💡Firm Yield

Firm yield is the maximum amount of water a source can supply during the worst possible drought. It's a critical concept in water management, as it determines the reliability of a water supply. The video uses the example of a river with no firm yield to illustrate the need for reservoirs to ensure a dependable water supply.

💡Water Storage

Water storage is the practice of holding water in a reservoir or other facility for later use, typically to manage seasonal or annual variations in water availability. The video discusses how dams create reservoirs to store water, transforming inconsistent natural water supply into a more constant and reliable resource.

💡Hydropower

Hydropower is the use of water's energy to generate electricity, often through the flow of water in a river or the release of water from a reservoir. While not the main focus of the video, it mentions that dams and reservoirs also serve the purpose of storing water for hydropower, highlighting the multifaceted use of water resources.

💡Groundwater

Groundwater is water stored beneath the Earth's surface in aquifers. The video touches on groundwater as an alternative water source when surface reservoirs like Lake Mead are depleted. It also discusses how over-pumping of groundwater can reduce river flows, as seen in the case of the Beaver River in Oklahoma.

💡Climate Change

Climate change refers to long-term shifts in temperature and precipitation patterns. The video suggests that climate change may be contributing to the 'mega-drought' in the southwestern United States, altering historical precipitation patterns and affecting the reliability of water reservoirs.

💡Desalination

Desalination is the process of removing salts and minerals from saline water to make it suitable for human use. The video briefly mentions desalination plants as a potential, though expensive, solution for water scarcity, converting seawater or brackish water into freshwater.

💡Water Conservation

Water conservation involves using water resources wisely to prevent waste and ensure availability. The video implies that as reservoirs dry up, utilities might invest in conservation efforts to encourage customers to use less water, which can help stretch limited supplies.

💡Economic Impact

The economic impact of water scarcity is a significant theme in the video. It discusses how the drying of reservoirs can lead to higher water prices, increased costs for agriculture, and the need for more expensive water sources, all of which affect the economy and cost of living.

Highlights

In June 2022, Lake Mead reached an all-time low of 175 feet below full, revealing rusted debris and human remains.

Many major water reservoirs in the western United States are critically low, including Lake Powell and California's Lake Oroville and Lake Shasta.

The variability in natural water supply is addressed by dams and reservoirs to ensure consistent water availability.

The concept of 'firm yield' is crucial for city planners, referring to the maximum water supply during the worst drought.

Reservoirs are built to store excess water during high flow periods to supplement dry periods, ensuring a reliable water supply.

Dams take advantage of natural topography to create large-scale water storage facilities without the need for massive tanks.

Reservoirs can increase the firm yield of a water source by storing water for multiple dry years, reducing the impact of droughts.

Dams serve multiple purposes beyond water supply, including agriculture, hydropower, recreation, and flood control.

Reservoirs should sometimes be empty to ensure good stewardship of water resources during periods of drought.

Engineers and planners face challenges in predicting the worst-case scenario drought due to limited historical data and changing climate patterns.

When reservoirs run out of water, it's often due to a combination of factors including climate change, overuse, and poor planning.

The economic impact of drying reservoirs is significant, leading to increased costs for water infrastructure and potential changes in land use.

In 2019, states using the Colorado River signed a drought contingency plan to manage water scarcity.

The case of Optima Lake Dam in Oklahoma illustrates the consequences of overestimating water availability and the impact of groundwater pumping.

Water management is a complex field that requires interdisciplinary knowledge and practical experience.

Brilliant.org/PracticalEngineering offers interactive learning in STEM fields, including courses on everyday physics.

Transcripts

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.

This video is sponsored by Brilliant,  

the best way to learn math and science  through problem solving. More on them later.

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.

Drought seems really simple when you’re just  looking at the level in a reservoir, but I hope  

this video helped you appreciate the technical  complexity in developing and managing water  

supply for a large area that involves hydrology,  geology, meteorology, climatology, and of course  

a lot of civil engineering. In fact, I’ve found  through everything I do that the all the best and  

most important projects combine the expertise and  knowledge of lots of different fields of study.  

And the way you get exposed to those different  fields isn’t by reading or watching videos.  

It’s by doing the thing, coding the program, by  building the project. That’s why I’m so thankful  

to have Brilliant as the sponsor of today’s video.  Brilliant is a learning platform for science,  

technology, engineering and math, that is super  interactive. There are courses on logic, computer  

science, math, and actually my favorite is this  one called the Physics of the Everyday that just  

pulls back the scientific curtain on aspects of  the world that you only learned about in grade  

school (for example, one of my favorite obsessions  - the weather). If that sounds interesting to you,  

go try Brilliant yourself completely free at  brilliant.org/PracticalEngineering. You don’t  

pay anything to sign up, and the first 200  people that use the link will get 20% off an  

annual premium subscription. Thank you for  watching, and let me know what you think.