There are great out wash plains below the volcanoes- there is always meltwater rushing to the sea and the outwash will have streams and rivers that are present all year round. The bridges that cross them are engineered to resist a massive outpouring of water, and the approaches are designed so that the road beds can be quickly cut and allow the water to flow around the bridge - easier to fix the road than replace the bridge.
It was really amazing to drive across these vast expanses. Some are miles and miles of gravel. Others are farmed. Once upon a time masses of water swept across huge areas. Some have been untouched for millennia. When Eyafjallajokull went up in 2010 the world knew about the ash that grounded the airplanes. But after the planes flew again the water came down, as it always does. This time there was warning and people were evacuated.
It takes a special kind of people to live underneath a volcano!