Disappear Gradually Smaller Petered Loop Peter River Upstream

1. We followed the river upstream as it got smaller and smaller until it finally petered out.
2. The members of the club got together every week until they began to lose interest. At first, just a few people stopped coming, but eventually they all petered out.

The expression does not mean to disappear gradually as in fade, but to disappear slowly in terms of quantity or size.