How do hydroelectric dams affect river systems?

A river is an extremely dynamic system that moves nutrients and minerals from the uplands to the sea while offering a vast variety of habitats and food.

In a hydroelectric scheme on a river, the natural flow (and variation in it) will typically (though not always) be stopped by a dam in order to generate power that essentially alters the nature of the river; the flow rate will then be carefully regulated to produce electricity when demand (and profit) is highest and will no longer have the constant variation (both annual and chaotic variation in amplitude) of a 'natural' river.

Some power companies have tried to mitigate this effect by providing "fish ladders" of varying design and success. The dam will also cause nutrients and silt to settle out, changing the distribution of both downstream and making it harder (or impossible) for spawning fish to climb higher in the river system to the habitat they require (often sand/gravel in my limited experience).

Lastly, grease, oil, and other materials used in the turbines, valves, and vanes that control water flow and extract energy from it will all contribute to pollution in HEP systems.

Hydroelectric dams can disrupt the natural flow of rivers, affecting sediment transport, fish migration, and nutrient distribution. They can also lead to changes in water temperature and dissolved oxygen levels, impacting aquatic ecosystems downstream. Additionally, dam construction can result in habitat loss and alteration, affecting the biodiversity of the river system.