What happens when reservoirs dry up? - Supply engineering (2023)

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

In June 2022, the water level in Lake Mead, the largest reservoir in the United States formed by the Hoover Dam, reached a record low of 175 feet or 53 meters below its maximum level, levels not seen since the dam was built. The lake was first filled in in the 1930s. Rusty shipwrecks, shipwrecks and even human remains come to the surface as the water level drops. Lake Mead is not alone. In fact, this is just a drop in the ocean. Many of the largest reservoirs in the western United States are running at very low levels, and the summer of 2022 has just begun. Lake Powell, in the upper reaches of Lake Mead on the Colorado River, is at its lowest level ever. Lake Oroville (famous for a massive spill) and Lake Shasta (two of California's largest reservoirs) reached critical levels. Utah's combined reservoirs are less than 50% full. even manyThe westernmost Texas ReservoirVery low in summer.

(Video) The Hidden Danger in Reservoirs Drying Up and Refilling so Fast

People use water at a more or less constant rate, but nature provides it in the form of unpredictable rain or snow that varies with the seasons and often varies significantly between seasons. If the water droplets are too far apart, we call it a drought. and in the leasta studyThe past two decades are estimated to have been the driest in over a thousand years in the southwestern United States, resulting in what is known as a "megadrought". Dams and reservoirs are one of the solutions to the dramatic changes in the natural water supply. But what happens when they stop filling up, or (in the case of an Oklahoma lake) what happens when they never fill up? My name is Grady and this is Applied Engineering. In today's program we talk about water supply and storage reservoirs.

The absolute need for water requires urban planners to always assume the worst. If you've had a dry year (or even a dry day), you can't just sit back and wait for the rain to come back. Therefore, the biggest problem in the development of new water sources is stable production. This is the maximum amount of water the spring can provide during the worst drought. Here is an example to make it clearer:

Imagine you are the director of public works for a new city. To keep your residents hydrated and clean, build a pumping station on a nearby river to collect water and send it to a treatment plant where it can be purified and distributed. The flow of this river is not constant. The currents are strong in the spring when the snow melts and run off the mountains, but slow down in the summer when the snow melts and the rain is more scattered. In very dry years, when the snow cover is thin, the river dries up almost completely. In other words, the stream has no fixed income. Water in any quantity is not a reliable supply. Of course, water is available most of the time, but most of the time it is not enough to meet basic human needs. So what do you do? One option is to store some excess water to keep the pumps running and the taps flowing during dry periods. However, the amount of storage is important.

Clean wells at water treatment plants or elevated water towers typically hold about a day's worth of water. These types of tanks are designed to smooth out variations in demand throughout the day (I have a video on the subject), but they don't add much to the reliability of the water source. Water towers are of little use if the river runs dry for more than a day at a time. To do this, increase your storage capacity by an order of magnitude (or two). This is why we build dams to create reservoirs, which in some cases hold billions of gallons or tens of thousands of liters at a time, in an incomprehensible (almost unfathomable) capacity. You will never be able to build a pond to hold that much liquid, but building a cistern over a river valley will allow water to fill the entire landscape like a bathtub. The dam takes advantage of nature's topography to form a simple but monumental water reservoir.

(Video) What Happens When You Remove a Dam

Let's build a small reservoir on your city's river and see how it can change the reliability of your water supply. If the reservoir is small, it will be filled with water most of the year. Any water not stored flows downstream as if the reservoir never existed. But in the summer, when the river's flow begins to drop, the reservoirs can be released to replenish supplies. In the dry years, there is still a chance that you will not store much water for the summer, but you will still have more than zero water, which means that your supply has a constant production and you can commit to supply a safe amount of water, even in the most unfavorable conditions, able to supply, approximately equal to the average flow in a dry year.

Now let's imagine that you build a larger dam to increase the size of the reservoir so that it can hold more than enough supplies for one season. Instead of just covering the deficit of the driest months, it can now cover the deficit of one or more dry years. Constant water source production increases further, approximating the long-term average of the river flow and completely eliminating the idea of ​​drought, turning all the inconsistent rain and snow into a completely constant supply. Furthermore, any increase in reservoir capacity will not increase yield. After all, a reservoir doesn't create water, it just stores what it already has.

Of course, dams do more than just store water for cities that need a constant supply of water for their citizens. They also store water for agriculture and hydropower, whose demand is more flexible. As a recreational destination, the reservoir has fueled a large-scale tourism economy. Some reservoirs are built only to supply cooling water to power plants. In addition, many dams are built larger than necessary to conserve water, so they can also absorb large floods (even when the reservoirs are full). Each reservoir has operational guidelines for when, where and under what conditions water can be withdrawn or released, and no two reservoirs are the same. However, I explain all this to clarify an important point: empty reservoirs are nothe shalla bad thing.

Building dams is expensive. They use a lot of public resources. They are dangerous structures that must be monitored, maintained and repaired with vigilance. In many cases, they have a significant impact on the natural environment. In short, we don't want to build more dams than we need. Empty reservoirs can create a negative public image. The dry lakebed is an ugly sight, and the "ring of bathtubs" around Lake Mead is a stark reminder of the water shortage in the American Southwest. However, not utilizing all the available storage capacity can be seen as a lack of good dam management, meaning that the reservoir must sometimes be empty. Why build it so big if the stored water won't be used during a drought? Storage is what it's all about... except there's one more thing to discuss:

(Video) How Wells & Aquifers Actually Work

Engineers and planners don't really know what the worst drought scenario will be over the life of a reservoir. In an ideal world, we could analyze thousands of years of historical water flow records to see how long a drought in a given body of water might last. In fact, some rivers have flow gauges that have struggled to collect data for over a century, but most do not. Therefore, planners must make a number of assumptions and use indirect sources of information when assessing the performance of new water supply reservoirs. But even if we could base our design on long-term histories, there is still another problem. There are no rules indicating that future climates on Earth will resemble past climates, and indeed there is reason to believe that the long-term average water flow in many parts of the world, as well as many other direct climate indicators, is changing. In this case, it makes sense to be concerned that the reservoir is running dry. As stated, reservoirs do not produce water, so if the total amount delivered to the watershed through precipitation declines over time, fixed reservoir production also declines.

Which brings me to the question of the entire video: what happens when the reservoir runs out of water? This is a rather complex issue, not only because water suppliers and distributors are relatively independent and decentralized from each other (capable of making very different decisions when faced with shortages), but also because impacts can occur over long periods of time. Most utilities have long-term plans that analyze future supply and demand, allowing them to develop new supplies or implement protective measures before customers face emergencies. Barring a major failure by the government or public administration, it is unlikely that one day you will turn on the faucet and there will be no running water. In fact, water supply is primarily an economic problem. Instead of saying we've run out of money, we just get it in more expensive ways. Utilities are spending more money on infrastructure, such as pipelines to bring water from places where it is abundant, wells that can extract groundwater sources, and even desalination plants that can convert sources of brackish water or even water from the sea into freshwater sources. Alternatively, utilities can invest in advertising and various conservation efforts to persuade customers to use less. Regardless, these costs are passed on to taxpayers and others.

For some people, such as those living in cities, higher water prices may be worth living in an otherwise inhospitable climate. For others, especially farmers, rising water costs could wipe out their profits and force them to temporarily or permanently land on their fields. So while the drying up of a reservoir may not be an emergency for most people, the effects will rub off on everyone through increased water rates, increased food costs and a host of other effects. Therefore, many see what is happening in the American Southwest as an unspoken "slow train wreck".

In 2019, all states that use water from the Colorado River signed ondrought emergencyIncluding plans to reduce use starting in Arizona and Nevada. These cuts will force farmers to rely on expensive and limited groundwater sources. Finally, irrigated agriculture in Arizona and Nevada may be a thing of the past. There is no doubt that the climate of the American Southwest is changing, with years warmer and drier than at any time in history. It is difficult to relate cause and effect to such sweeping and dramatic changes in long-term weather patterns, but I have an example of an empty reservoir and there is no doubt why it is drying up.
In 1978, the US Army Corps of Engineers built the Optima Lake Dam on the Beaver River in Oklahoma. The dam is an earth dam 120 feet (or 37 meters) high and over 3 miles or 5 kilometers long. The Beaver River in Oklahoma has historically averaged about 30 cubic feet per second, or nearly a cubic meter, and the river has even experienced some major floods that sent a lot of water downstream. But things are clearly changing rapidly during the construction of the dam. It turns out that much of the Beaver River's flow comes from headwaters, areas where groundwater rises to the surface. During the 1960s and 1970s, urban and agricultural groundwater pumping reduced aquifer levels in the region, cutting off the flow of Beaver Creek. As a result, when the construction of this massive earthen dam was completed, the reservoir was never filled. Now Optima Lake Dam sits in the Oklahoma Panhandle, mostly high and dry, never above 5% capacity, a monument to climate misunderstandings and to engineers, water planners and anyone learning a lesson in understanding the challenges we face in the water. sector. The future is drier.

(Video) Hoover Dam | All the Secrets of the Engineering Wonder

(Video) What Happens When a Reservoir Goes Dry?


What happens when reservoirs dry up? ›

If water levels fall too low in these reservoirs, they could reach “dead pool.” That's the point when water would be effectively blocked from passing Glen Canyon Dam (on Lake Powell) and Hoover Dam (on Lake Mead) — cutting off the lifeline to some 25 million people who get water from Lake Mead.

What happens when reservoirs are full? ›

If the reservoirs overflow, water can spill into nearby rivers and rush down, potentially flooding small towns or campsites. Some of the water will be lost, a situation that isn't ideal in a drought.

What happens to the water in a reservoir? ›

Evaporation is a common problem with reservoirs. In wet areas, the water that evaporates often falls again as rain. But in hot, dry areas, evaporation can result in a huge loss of water. The level of reservoirs in desert areas can drop 1.5 meters (five feet) in a single year because of evaporation.

How is water supplied from reservoir? ›

In many big cities, reservoirs are located far away, and the water is brought to the city through aqueducts. If you have ever tried to stop the flow of water from a hose or pipe with your thumb, or if you have seen a hose or a pipe burst, you know that water pressure is very high.

What reservoir is drying up? ›

In 2021 California shut down a power station at Lake Oroville, one of the state's largest reservoirs, when water levels fell below what is needed for electricity generation. The plant can usually provide enough power for 80,000 homes. Lake Powell, on the border of Utah and Arizona, is inching towards a similar fate.

What do you think will happen if freshwater dries up? ›

When waters run dry, people can't get enough to drink, wash, or feed crops, and economic decline may occur. In addition, inadequate sanitation—a problem for 2.4 billion people—can lead to deadly diarrheal diseases, including cholera and typhoid fever, and other water-borne illnesses.

What are the problems with reservoirs? ›

As sediment accumulates at the face of a dam, it covers the dam's valves, making them inoperable, thereby reducing the ability to operate the dam to reduce flood risk. Additionally, as sediment accumulates behind the dam, the water conservation capacity of the reservoir behind the dam is reduced.

Why do reservoirs get drained? ›

Throughout the late fall and winter, most are required to release water if they get too full, sometimes emptying out almost by half. That's because the empty space is crucial if an intense storm hits. Reservoirs collect runoff and prevent it from flooding downstream cities.

Why is reservoir water important? ›

Answer: A reservoir is an artificial lake created in a river valley by the construction of a dam. The most critical purpose of reservoirs is flood risk management. Reservoirs collect water during times of high rainfall, reducing flood risk, and then release the water slowly over the following weeks and months.

What will happen if Lake Mead completely dries up? ›

What happens if Lake Mead dries up forever? If Lake Mead were to run out of water, the Hoover Dam would no longer be able to generate power or provide water to surrounding cities and farms. The Colorado River would essentially stop flowing, and the Southwest would be in a major water crisis.

What is the life of a reservoir? ›

The term 'life of reservoir' as loosely used denotes the period during which whole or a specified fraction of its total or active capacity is lost. In calculating this life, the progressive changes in trap efficiency towards the end of the period are commonly not considered.

What will happen if the Colorado River dries up? ›

If the water levels dip much lower, the Colorado's northernmost reservoir won't have enough in the tank to both fill Lake Mead downstream and generate any hydropower, which would have devastating effects on the electricity grid in the western US.

What are problems reservoirs cause? ›

Slow-moving or still reservoirs can heat up, resulting in abnormal temperature fluctuations which can affect sensitive species. This can lead to algal blooms and decreased oxygen levels. Other dams decrease temperatures by releasing cooled, oxygen-deprived water from the reservoir bottom.


1. The Real Reason Lake Mead is Drying Up
2. When Silt Fills Your Water Reservoir
3. Dried-Up Reservoirs: Are They a Real Threat? Find Out Here!
(Extreme Engineering)
4. The Simple Genius of NYC’s Water Supply System
(Wendover Productions)
5. How to build a dam
(Interesting Engineering)
6. How Things Work : How Do Water Springs Work?


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