The city's great waterfall, now stopped, its channel about to carry lava instead.
From the tower stairs during the siege, Carl notes that the waterfall across the way has stopped. He knows what that means: the slow plod of The Lava Flow will soon replace it.
Donut confirms the timing from her perch — about two minutes until the river of lava falls over the edge.
A dry, silent waterfall across the ring of the funnel city, smoke drifting through where water used to fall.
The stilled landmark signaling the lava's approach.