There is a contrast here between the high towers built by man and God's high mountains and lofty hills. The collapsing towers were probably those of the proud Assyrian oppressors at the end of Israel's long captivity. The context of this passage concerns God's restoration of his repentant people. Mountains were usually barren. Streams suggest blessing and fruitfulness for a farming culture.
Streams are representative of spiritual renewal in scripture.
By this he meant the Spirit, whom those who believed in him were later to receive. Up to that time the Spirit had not been given, since Jesus had not yet been glorified. When the pride of man falls, streams of water will flow. Interestingly, in 1994 streams of renewal began to pour through out the world. This reference is God's response to the national prayer of an ancient Head of State.