<p>You're correct that simply putting a dam in place, once its lake is filled, doesn't change the <em>average</em> flow downstream by more than a few percent (those few percent <a href="https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-15-00224.1" rel="noreferrer">can be lost</a> to increased evaporation). However,</p>

<ul>
<li>Many dam systems are used to abstract water from the river for irrigation and other uses, sometimes on a very large scale. This does reduce the average flow downstream.</li>
<li>Depending on the site, filling the lake might take a year or more, which could have severe consequences on people downstream.</li>
<li>More generally, dams allow regulation of the downstream flow. This is a major reason for their construction, in areas that want to smooth out a flood/drought cycle. But it means that downstream water supply is then under the control of the dam operators, which may be objectionable to people downstream - especially when the river crosses international borders. The water must be released eventually, but the upstream operator could choose to (for example) withhold it for the growing season and cause crop failure that year.</li>
</ul>

<p>For all of these reasons, river management is a common subject of international treaties. I don't know the details of this particular case involving Ethiopia and Egypt, but I imagine that the main concern is probably about water abstraction. If Ethiopia takes water for drinking, irrigation, etc., then that's less water left for Egypt.</p>

CC BY-SA 4.0
2019-10-24T17:48:12.933Z
Evaporation is 60% for Lake Tahoe [RESERVOIR EVAPORATION IN
THE WESTERN UNITED STATES, p. 171](https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-15-00224.1).
Keith McClary
CC BY-SA 4.0
2019-10-24T22:06:08.700Z
I'm pretty sure that article doesn't say that. However, it is a helpful article, and Figure 4 suggests that around 3-5% of the water leaving a large reservoir or lake does so through evaporation. Which is quite a lot. Thanks for the link! I've added this info to the answer.
Semidiurnal Simon
CC BY-SA 4.0
2019-10-25T01:38:08.970Z
[The Truckee River is the lake's only outlet, flowing northeast through Reno, Nevada, into Pyramid Lake which has no outlet. It accounts for one third of the water that leaves the lake, the rest evaporating from the lake's vast surface.](https://en.wikipedia.org/wiki/Lake_Tahoe#Geography)
Keith McClary
CC BY-SA 4.0
2019-10-25T01:39:35.110Z
Much less from the Renaissance Dam: [Evaporation of water from the reservoir is expected to be at 3% of the annual inflow volume of 48.8 km3 (11.7 cu mi), which corresponds to an average volume lost through evaporation of around 1.5 km3 (0.36 cu mi) annually. This was considered neglectable by the IPoE. For comparison, Lake Nasser in Egypt loses between 10–16 km3 (2.4–3.8 cu mi) annually through evaporation.](https://en.wikipedia.org/wiki/Grand_Ethiopian_Renaissance_Dam#Siltation,_evaporation_and_irrigation)
Keith McClary
CC BY-SA 4.0
2019-10-25T15:41:46.533Z
@KeithMcClary Lake Tahoe is primarily a natural lake, though the dam raises the level by 10 feet.  If you want to take another extreme example, Crater Lake is 100% outflow by evaporation and groundwater.
DavePhD
CC BY-SA 4.0
2019-10-25T17:24:07.297Z
@KeithMcClary OK, I missed that in the report (and I had misread Figure 4). But as DavePhD noted, Lake Tahoe does not appear to be very typical of a lake behind a dam. If the flow out of a lake is very small compared to the size of the lake (perhaps because of where you chose to put the dam, or because of choices in operation of the dam), then clearly the proportion of water loss that is by evaporation, rather than river flow, will be higher. At an extreme, the evaporative loss from a lake with no outflow is nearly 100%.
Semidiurnal Simon
CC BY-SA 4.0
2019-10-24T17:07:42.203Z
2019-10-24T22:07:07.367Z
2019-10-24T22:07:07.367Z
Semidiurnal Simon
<p>Once a dam has been constructed in a river, the <em>natural flow</em> of water will be disrupted.</p>
<p>You correctly state that initially there will be a period required for the dam to fill. Until then, little of no water will flow beyond the dam.</p>
<p>Dams are created for one or more of four reasons:</p>
<ol>
<li>To supply potable water to a population of humans for domestic or
industrial purposes.</li>
<li>For agricultural purposes.</li>
<li>To generate hydro-electricity.</li>
<li>Flood control</li>
</ol>
<p>Water released from a dam prior to it being full may be to restore some flow for environmental reasons, because hydro electricity may need to be generated or it may be needed for agricultural reasons.</p>
<p>To protect dams from overfilling and damaging the dam wall spillways are generally constructed to allow the excess water to flow beyond the dam. For dams without a spillway, the dam gates will be opened to reduce the amount of water in the dam to protect the dam from failure.</p>
<p>One of the issues with dams is they prevent sediment carried by the river to travel the full length of the river. This can be detrimental to the natural replenishment of some farmlands of nutrient rich sediment and it is detrimental to the replenishment of nutrients to fish breeding regions near the mouths of rivers. This happened when the Egyptians built the <a href="https://en.wikipedia.org/wiki/Aswan_Dam" rel="nofollow noreferrer">Asswan Dam</a>.</p>
<p>The other issue with dams is that people downstream of the dam cannot rely of the natural flow of the river, in terms of timing of flows and quantities of flow. This is the main concern for Egypt. It relies heavy on the Nile River as its main source of fresh water for hydro-electricity generation, agriculture and water for people to drink.</p>

CC BY-SA 4.0
2019-10-25T11:09:24.943Z
It's a complex picture. I remember an experiment somewhere in the US where the constant flow below the dam caused sediment to *accumulate* in river bends, damaging certain wildlife (unable to lay eggs IIRC). By occassionaly letting large amounts of water through they managed to flush out the sediment again. Sorry, couldn't find the actual info agian.
Jan Doggen
CC BY-SA 4.0
2019-10-25T11:40:01.480Z
Dams are also created as a weapon of war.  The controlling nation could hypothetically release a flood on the other nation.
emory
CC BY-SA 4.0
2019-10-25T11:49:42.763Z
@emory has this ever been done?
Josef
CC BY-SA 4.0
2019-10-25T19:12:52.067Z
@Josef ["One of the most notable recent events in the river’s troubled history occurred in June 1938, when the Nationalist Chinese Army diverted the river to block invading Japanese troops."](https://www.earthmagazine.org/article/june-9-1938-huang-he-diversion-largest-act-environmental-warfare-history)
Keith McClary
CC BY-SA 4.0
2019-10-25T20:05:56.133Z
@Josef Not only has it been done, https://en.wikipedia.org/wiki/Imnam_Dam, but there is at least one dam built to defend against such an attack.  Because the https://en.wikipedia.org/wiki/Peace_Dam has no reservoir, it could not possibly supply potable water, used for agricultural purposes, or generate hydro-electricity.  Its only reason for existence is military defense.
emory
CC BY-SA 4.0
2024-11-07T15:53:12.763Z
4. Flood control. You identify flow issues as possible *consequences*. But mitigating floods should be added to the list of reasons why dams are *created*.
Bob Stein
CC BY-SA 4.0
2024-11-08T06:51:35.403Z
@BobStein: Good point. I'll add it to the answer. Thanks.
Fred
CC BY-SA 4.0
2019-10-24T16:19:23.723Z
2024-11-08T23:24:44.027Z
2024-11-08T23:24:44.027Z
Fred
<p>There is a conflict between Egypt and Ethiopia, because the latter wants to build a dam on the Nile river. Citing <a href="https://www.aljazeera.com/news/2019/10/ethiopia-nobel-prize-winning-leader-issues-warning-dam-191022141801474.html" rel="noreferrer">this article</a>:</p>

<blockquote>
  <p>[Egypt] fears the Nile dam will reduce its share of the river and leave the
  country with dwindling options as it seeks to protect its main source
  of fresh water.</p>
</blockquote>

<p>What do they mean by "reduce its share of the river"? As far as I understand, the dam will create a reservoir, which will initially divert some water to be filled, but once that is done, the flow of the river should be the same. Is Egypt worried about this initial reduction, or do dams somehow reduce the flow of a river?</p>

<p>If so, what is the mechanism? I would guess that this is due to increased evaporation from the river, since the reservoir has a bigger surface than the usual river, but then again, it also collects more rain.</p>

<hydrology><rivers><dams>
Do dams reduce the flow of river downstream?
Martin Drozdik
# Do dams reduce the flow of river downstream?

(Asked by: username_0 on 2019-10-24)

There is a conflict between Egypt and Ethiopia, because the latter wants to build a dam on the Nile river. Citing [this article](https://www.aljazeera.com/news/2019/10/ethiopia-nobel-prize-winning-leader-issues-warning-dam-191022141801474.html):

> [Egypt] fears the Nile dam will reduce its share of the river and leave the
> country with dwindling options as it seeks to protect its main source
> of fresh water.

What do they mean by "reduce its share of the river"? As far as I understand, the dam will create a reservoir, which will initially divert some water to be filled, but once that is done, the flow of the river should be the same. Is Egypt worried about this initial reduction, or do dams somehow reduce the flow of a river?

If so, what is the mechanism? I would guess that this is due to increased evaporation from the river, since the reservoir has a bigger surface than the usual river, but then again, it also collects more rain.

**Tags:** hydrology, rivers, dams


---

## 2 answers

### Answer by username_4 on 2019-10-24. Score: 20 | Accepted answer

Once a dam has been constructed in a river, the *natural flow* of water will be disrupted.

You correctly state that initially there will be a period required for the dam to fill. Until then, little of no water will flow beyond the dam.

Dams are created for one or more of four reasons:

1. To supply potable water to a population of humans for domestic or
   industrial purposes.
2. For agricultural purposes.
3. To generate hydro-electricity.
4. Flood control

Water released from a dam prior to it being full may be to restore some flow for environmental reasons, because hydro electricity may need to be generated or it may be needed for agricultural reasons.

To protect dams from overfilling and damaging the dam wall spillways are generally constructed to allow the excess water to flow beyond the dam. For dams without a spillway, the dam gates will be opened to reduce the amount of water in the dam to protect the dam from failure.

One of the issues with dams is they prevent sediment carried by the river to travel the full length of the river. This can be detrimental to the natural replenishment of some farmlands of nutrient rich sediment and it is detrimental to the replenishment of nutrients to fish breeding regions near the mouths of rivers. This happened when the Egyptians built the [Asswan Dam](https://en.wikipedia.org/wiki/Aswan_Dam).

The other issue with dams is that people downstream of the dam cannot rely of the natural flow of the river, in terms of timing of flows and quantities of flow. This is the main concern for Egypt. It relies heavy on the Nile River as its main source of fresh water for hydro-electricity generation, agriculture and water for people to drink.

> **Comments:**
> **username_5**: It's a complex picture. I remember an experiment somewhere in the US where the constant flow below the dam caused sediment to *accumulate* in river bends, damaging certain wildlife (unable to lay eggs IIRC). By occassionaly letting large amounts of water through they managed to flush out the sediment again. Sorry, couldn't find the actual info agian.  
> **username_6**: Dams are also created as a weapon of war.  The controlling nation could hypothetically release a flood on the other nation.  
> **username_7**: @username_6 has this ever been done?  
> **username_2**: @username_7 ["One of the most notable recent events in the river’s troubled history occurred in June 1938, when the Nationalist Chinese Army diverted the river to block invading Japanese troops."](https://www.earthmagazine.org/article/june-9-1938-huang-he-diversion-largest-act-environmental-warfare-history)  
> **username_6**: @username_7 Not only has it been done, https://en.wikipedia.org/wiki/Imnam_Dam, but there is at least one dam built to defend against such an attack.  Because the https://en.wikipedia.org/wiki/Peace_Dam has no reservoir, it could not possibly supply potable water, used for agricultural purposes, or generate hydro-electricity.  Its only reason for existence is military defense.  
> **username_8**: 4. Flood control. You identify flow issues as possible *consequences*. But mitigating floods should be added to the list of reasons why dams are *created*.  
> **username_4**: @username_8: Good point. I'll add it to the answer. Thanks.  

---

### Answer by username_1 on 2019-10-24. Score: 14

You're correct that simply putting a dam in place, once its lake is filled, doesn't change the *average* flow downstream by more than a few percent (those few percent [can be lost](https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-15-00224.1) to increased evaporation). However,

* Many dam systems are used to abstract water from the river for irrigation and other uses, sometimes on a very large scale. This does reduce the average flow downstream.
* Depending on the site, filling the lake might take a year or more, which could have severe consequences on people downstream.
* More generally, dams allow regulation of the downstream flow. This is a major reason for their construction, in areas that want to smooth out a flood/drought cycle. But it means that downstream water supply is then under the control of the dam operators, which may be objectionable to people downstream - especially when the river crosses international borders. The water must be released eventually, but the upstream operator could choose to (for example) withhold it for the growing season and cause crop failure that year.

For all of these reasons, river management is a common subject of international treaties. I don't know the details of this particular case involving Ethiopia and Egypt, but I imagine that the main concern is probably about water abstraction. If Ethiopia takes water for drinking, irrigation, etc., then that's less water left for Egypt.

> **Comments:**
> **username_2**: Evaporation is 60% for Lake Tahoe [RESERVOIR EVAPORATION IN THE WESTERN UNITED STATES, p. 171](https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-15-00224.1).  
> **username_1**: I'm pretty sure that article doesn't say that. However, it is a helpful article, and Figure 4 suggests that around 3-5% of the water leaving a large reservoir or lake does so through evaporation. Which is quite a lot. Thanks for the link! I've added this info to the answer.  
> **username_2**: [The Truckee River is the lake's only outlet, flowing northeast through Reno, Nevada, into Pyramid Lake which has no outlet. It accounts for one third of the water that leaves the lake, the rest evaporating from the lake's vast surface.](https://en.wikipedia.org/wiki/Lake_Tahoe#Geography)  
> **username_2**: Much less from the Renaissance Dam: [Evaporation of water from the reservoir is expected to be at 3% of the annual inflow volume of 48.8 km3 (11.7 cu mi), which corresponds to an average volume lost through evaporation of around 1.5 km3 (0.36 cu mi) annually. This was considered neglectable by the IPoE. For comparison, Lake Nasser in Egypt loses between 10–16 km3 (2.4–3.8 cu mi) annually through evaporation.](https://en.wikipedia.org/wiki/Grand_Ethiopian_Renaissance_Dam#Siltation,_evaporation_and_irrigation)  
> **username_3**: @username_2 Lake Tahoe is primarily a natural lake, though the dam raises the level by 10 feet.  If you want to take another extreme example, Crater Lake is 100% outflow by evaporation and groundwater.  
> **username_1**: @username_2 OK, I missed that in the report (and I had misread Figure 4). But as username_3 noted, Lake Tahoe does not appear to be very typical of a lake behind a dam. If the flow out of a lake is very small compared to the size of the lake (perhaps because of where you chose to put the dam, or because of choices in operation of the dam), then clearly the proportion of water loss that is by evaporation, rather than river flow, will be higher. At an extreme, the evaporative loss from a lake with no outflow is nearly 100%.  

---

