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There is a conflict between Egypt and Ethiopia, because the latter wants to build a dam on the Nile river. Citing this article:

[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.

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    $\begingroup$ The dam would allow the controlling country to determine when and how much water is released downstream. Messing with that would have consequences to all downstream users of the river. I don't believe that the rain falling on the reservoir would accumulate any more water than rain falling in the catchment basin, so I expect the evaporation rate to increase overall, but I don't have any literature to support that view $\endgroup$ – Friddy Oct 24 at 16:03
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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 three 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.

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.

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.

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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 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.

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