The conquistador responsible for taking over the city of tenochtitlán was

The correct answer is Hernán Cortes.

Hernan Cortes (1485-1547) was a Spanish Conquistador (or conqueror) who led the forces which caused the fall of the Aztec Empire and its capital, the city of Tenochtitlán. The siege and fall of this city took place in 1521, after systematic manipulations of the local forces and due to very valuable informants such as La Malinche. She was an indigenous woman who became the interpreter and companion of Hernan Cortes and revealed crucial information to him.  

what are local authorities doing to fix the problem

explanation:

these civilizations were located around fertile rivers, where they could produce an abundance of crops