The answer to this question is actually very simple: because they need cooling-water. Not only nuclear power plants need cooling-water for that matter. Also “classic” power plants (using gas, coal or fuel oil as heat source) need it and are therefore situated near rivers or canals.
Both power plant types produce electricity in the same way: high quality steam makes the turbines run and those turbines supply electricity to the network through the alternator. Once the quality of that steam has too much decreased, it cannot be of use for electricity production any more. It has to be cooled down till it turns back to water, that can be reused to produce high quality steam. This cooling takes place in the condensor and requires large quantities of cooling-water.
After having used the cooling-water in the (nuclear) power plant, it is returned to the river. It is obvious that this water does not contain any radioactive elements. As it has been used as cooling-water, it has of course been warmed up. The warmer the water, the lesser oxygen it contains, which is harmful for the fish and other fauna in the river. This is why the cooling-water generally is cooled down in the cooling towers, before its release into the river (see concerning FAQ).
When more (nuclear) power plants are situated along a river - like in Doel and Tihange - and when the outside temperature is rather high during a long period (like during the long hot summer of 2003), it may happen that, in spite of the cooling towers, the temperature at the cooling-water outlet exceeds the limits of the environmental regulations. In such case, one or more power plants cannot operate at full capacity.