The key event that is introduced in this paragraph is "the environmental impact of the Three Gorges Dam".
In writing, introducing an event means mentioning an event or issue that is essential in the text without explaining it completely. Instead, the event is mentioned for the reader to anticipate the topic of the text and this event is developed in the next paragraphs.
In the excerpt, the event that is introduced is "the dam has also caused significant environmental damage" because this seems to be an important event in the text and it is not being developed or explained immediately. Based on this, the key event introduced focuses on the environmental effects of the dam.
Learn more about Key event in: https://brainly.com/question/404286