YOGYAKARTA, Indonesia—Indonesia’s most active volcano erupted Monday with its biggest lava flow in months, sending a river of lava and searing gas clouds flowing 3.5 kilometers (more than 2 miles) down its slopes on the densely populated island of Java. The rumbling sound could be heard several miles away as Mount Merapi erupted, sending hot ash 600 meters (nearly 2,000 feet) into the sky. Ash blanketed nearby towns, but long-established evacuation orders are in place near the volcano, and no casualties were reported. It was Merapi’s biggest lava flow since authorities raised its danger level last November, said Hanik Humaida, the head of Yogyakarta’s Volcanology and Geological Hazard Mitigation Center. She said the lava dome just below Merapi’s southwest rim and the lava dome in the crater both have been active since the end of July. The southwest rim dome volume was estimated at 1.8 million cubic meters (66.9 million …