What influence did the aryans have on the people of indus valley ?

The Aryan have a big influence towards the Indus Valley People, which is the Dravidian. Before the Aryan came, the Dravidian has a very sophisticated technology that can be seen from the Mohenjo-Daro city which has sanitation system, etc. But, when the Aryan came with army they conquered the Indus Valley, and drove away the Dravidian. And the Aryan became the ruler of the Indus Valley. The Dravidian became slaves, etc. To maintain the purity of the Aryan, they create a cast system that divided the Indus Valley people into 4 major cast : Brahma (priest), Ksatria (King and Royals), Waisya (Traders), and Sudra ( Slaves, which is the Dravidian)