Complex sentence: After reaching Catalonia the Ebro Valley narrows and the river becomes constrained by mountain ranges and massive dams have been built in this area Mequinensa, Riba-roja, Flix.
Split into simpler sentences: After reaching Catalonia the Ebro Valley narrows and the river becomes constrained by mountain ranges. Massive dams have been built in this area, like the dams at Mequinensa, Riba-roja, Flix.