I think that the three gorges dam was both worth it, and not. I think this because it depends on the scale of the impacts you are taking into consideration.
On a smaller, more localised scale, the environmental, social, and even economic impacts are overall negative. Environmentally, the only advantage is that it helps to protects over 25000 hectares of farmland. The disadvantages include the landslides caused by the fluctuating water level, the decrease in fish stocks (impacting local fishermen) – including the extinction of the Yangtze river dolphin – and the slowing of the water flow, leaving more trash in the river, and decrease in water quality (higher silt concentration). Socially, there are more advantages, like the increase in jobs – garbage relief – and helping to control flooding. However, disadvantages are that more than 1.4 million people were forced to move, people are losing their homes due to landslides, causing widespread fear, and that more than 8000 historical sites and fertile agricultural land has been flooded. Economically, the only advantage is the tourism, which largely existed before the dam, while the biggest disadvantage is that the government is compensating many locals’ lost homes.
On a larger scale, one of all of China, I think that the impacts are overall positive. Environmentally speaking, the dam is powering 10% of China’s electricity – 28 cities – which used to be produced with coal and other fossil fuels, thus reducing the emissions, and possibly even smog. From a more social perspective, it’s reduced flooding, and with it, mortalities. Economically, the 50 million tonnes of coal is not having to be imported due to the construction and capacity of the dam. Now, the water level has risen, deepening the river so that larger ships can get through, increasing imports and exports out of China.