These valleys originally were streams that flowed to the south. The moraine formed a series of lakes in the valleys. Drainage was still to the south over the moraine until the glacier retreated far enough to the north to allow escape of the waters along the southern edge of the glacier.
The original waterways in these valleys were streams that flowed towards the south. The moraine created a chain of lakes within the valleys. It wasn't until the glacier moved significantly northwards that the waters could flow out of the valleys along the southern edge of the glacier.