According to my light reading after hearing this fact, one of the reasons rivers flow is because the ground below is saturated with water. Otherwise it would just seep in to the ground and flow through the soil. So what happens when you build a giant wall and stop that water from moving?
Well it puts a lot more pressure on everything beneath it. It can push soil and other parts of the ground it cracks in rocks and tectonic plates, making these cracks larger and putting a lot of stress on everything. Oh and don’t forget that we like to let the water pour out very very quickly (to produce electricity) and then fill the reservoir up again. These fluctuations of pressure create expansion and contraction that adds more stress in to the system.
When this stress and pressure builds up it can have catastrophic consequences. It’s thought that the Koyna Dam in Western India caused a 6.5 magnitude earthquake that killed 180 people. So yeah, it’s not just the damage to ecosystems that we need to think about when building these megastructures. They pose a real threat to people too.