DULUTH — The 100-plus inches of snow that fell on much of the Arrowhead region this winter are swelling North Shore waterfalls, aided by the area’s steep, jagged bedrock, formed a billion years ago when the continent split, lava flowed and massive glaciers sculpted the landscape. Both the temporary “pop-up” kind and the established falls that surge from spring snowmelt are expected to be “epic” in the coming weeks, “with record snowfalls in the forest,” said Linda Jurek, executive director of Visit Cook County. The organization in recent years has begun promoting the annual occurrence to drive traffic during the quieter spring tourism weeks, with plenty of resorts advertising “waterfall…