Complex sentence: The Idriss I dam is a large water impoundment structure built on the Moroccan Inaouen River in the year 1973; the Inaouen River is a tributary of the Sebou River.
Split into simpler sentences: The Idriss I dam is a large water impoundment structure built on the Moroccan Inaouen River in 1973. The Inaouen River is a tributary of the Sebou River.