Very quickly below the dam the flow of the river will be controlled by downstream parameters and by the volume of water in the open channel only. The energy recovered by the turbines is not 100% and the kinetic energy of the released water  during power generation could actually be greater than it would be before the dam was built but the system will soon return to flows controlled by channel flow physics which will depend on the shape and  slope of the riverbed and frictional characteristics of the bed material. Obviously just below the dam you will see some effect and there will be periods when flow velocity is reduced because water is being released from the dam at a lower rate than it is flowing into it.

In some situations water is released well  downstream of the dam or, worse still, into a different catchment. This can have serious effects on parts of the river.

I agree with Alan, but would like to add, that the VARIABILITY of the flow velocity will also increase below a dam as periods of high velocity (much water released) will alternate with periods of low flow. During low flow fine sediment may be depostied near the shore or between the gravel , which is not alway carried away by the following high discharge and may accumulate.

It makes a lot of diffence, which kind of power plant we are talking about: the lower the residual flow below a dam, the more damage to the ecosystem.

The short answer is: yes, and water velocity is not the only concern. Hydrological processes affecting river channel morphology will also be different, even water temperature may change - depending on on from where water is released.

Hope, this helps, kind regards

Irene

Dear Kamal, I also assess the environmental effects of Dam construction in our area, and Yes it definitely affect the river flow or velocity.  

Prior to dam construction, most natural rivers have a flow rate that varies widely throughout the year in response to varying conditions. Of course once constructed, the flow rate of the river below a dam is restricted. The dam itself and the need to control water releases for the various purposes of the particular dam result in a flow rate that has a smaller range of values and peaks that occur at times related to need rather than the dictates of nature. In cases where the entire flow has been diverted for other uses, there may no longer be any flow in the original channel below the dam.

Best Regards,

Wilbert

Read more: Dams - Impact Of Dams - Water, River, Flow, and Sediment - JRank Articles http://science.jrank.org/pages/1942/Dams-Impact-dams.html#ixzz3WcTFZC9n

The variability of the flow rate over the whole year may be reduced below a dam, but short-term (hours/days) changes between high and low flow (and water velocities) may cause considerabel damage to the riverine biota below a dam. The greater the difference between high and low flow, the larger the damage.

kind regards

Irene

Irene and Wilbert are both correct that the dam will cause a change to both flow velocity and flow velocity variability but this is due to variations in volume in the river rather than the loss of energy to the turbines as implied by your question. Except in "run of the river"  hydro power plants the energy extracted by the turbines comes from the difference in head between the water level in the dam and the discharge point. This can be achieved by impounding the river to raise the head or using the natural topography to achieve the head (or a combination of both). If the topography is used to create the head the river is affected over a much greater distance than in the case of a dam with turbines just below the dam or inside it. The difference in head is then used to generate kinetic energy by getting a relatively small volume of water to move very quickly through the turbines but this does not come from the upstream kinetic energy of the river.

Thanks Alan

Your first answer seems convincing but in your second answer I think upstream, that is before the dam, we have only potential energy and this energy is converted at the turbines into kinetic energy. Assording to the first law of thermodynamics energy can neither be created nor destroyed but it can be transformed into other forms. So the electric power generated in the turbines is taken from the energy of the water i.e. the kinetic energy and so it is expected that the flow energy of the water is going to be reduced and then consequently the river flow velocity.

Actually the energy does not come from the kinetic energy of the river in the sense that the river water is moving at a given velocity and then releases energy as it slows. The energy come from the potential energy stored in the dam by the increased head of water. However I guess that at various points of the water cycle kinetic and potential energy are regularly interchanged and this is just one of them. The point is that once water Is abstracted into a leet or penstock I would no longer consider it to be part of the river  but part of the power generation system. Remember that the water is sometimes released into an entirely different catchment that it would normally be flowing in.

@Kamal: sorry, I got carried away by the eco-hydrology aspect and did not really answer your question.

Physics is not my field, but allow me to speculate: in a hydropowerplant most energy will be "lost" due to friction within or near the turbines - with the turbine, the tubes leading to and from the turbine, ect. However, in a natural river energy will also be "lost" due to the friction of the water against the  riverbed. In an impoundment this friction with the river bed is much lower than in a natural river. So in most situations this "natural" friction is probably much higher than the friction in the hydropowerplant plus the friction of the water int he impoundment, e.g. in most situations water velocity ought to increase. But to what extent? I guess it really depends on the situation...

kind regards

Irene

Irene is right. It does depend on the situation but in a typical "large dam" scenario with turbines within the dam wall the way to think about it is as follows:

If you had an extra dam at the upstream end of the impoundment the kinetic energy of the water within the impoundment would be zero. You can then imagine both dams being bypassed by the upstream flow.. If you then allow water to pass through the turbines in the downstream dam you would initially be extracting energy from the stored potential energy in the impoundment at the same rate as you could without the upstream dam. It would just depend on the head of water in the impoundment.  In a real situation the kinetic energy from the water flowing into the impoundment serves only to "top up" the potential energy within the impoundment.

It is the act of impounding the river and raising its water level with a dam that removes the kinetic energy from the flowing water not the act of passing water through a turbine. In fact as the water slows as it enters the impoundment much of the kinetic energy would be lost as heat.

Obviously as the impoundment is allowed to fill the volume of water passing through gate valves in the dam should be controlled initially so as to limit the downstream ecological harm. After generation is started you need to ensure that the total water released is enough to prevent ecological harm even when there is no abstraction.

This thought process needs to be modified in a "high head" situation where a small dam at the top of a mountain releases a smaller volume of water (at much higher pressure) to turbines at the bottom of a mountain but the logic is much the same. Here the length of river affected by the diversion of the water needs to be protected by a suitable flow through the dam.

Thank you Arash but the question is about the effect on the down stream velocity after extracting energy from the water as electricity. Will this extracted energy affect the water flow down stream?