The plug altered the course of the Passaic River, which had drained the pre-lake swamp. A range of mountains to the west of Morristown formed the western boundary of the new lake and the most easterly line of the Watchung Mountains became the eastern boundary. The tops of some of the Watchung range became islands in the great lake. Water that had vented through the Watchung range, or to its south, found a new path that altered the old drainage paths. When the plug collapsed, the river still was forced to travel north through the range before finding a new outlet near present-day Paterson where it could manage the eastern turn toward the sea.