Some are. Lahars that form during a volcanic eruption, called hot or primary lahars, travel and come to rest at an elevated temperature, while cold or secondary lahars are not tied to eruptions and have no elevated temperature.
Hot lahars start when eruption activity such as ash fall, pyroclastic flows, or lava flows and domes melts snow or ice, when very wet volcanic landslides liquefy, when an eruption breaks into a crater lake, or when heavy rain falls during an eruption. Snow- and ice-covered volcanoes are especially prone to them because melting supplies so much water. Cold lahars usually begin when heavy rain or snowmelt picks up loose volcanic deposits, and landslides, flooding or glacial outburst floods can also set them off, sometimes long after an eruption. Cold lahars are frequently smaller than hot ones. Either type is a mix of water, volcanic ash, rock fragments and ice that flows like wet concrete and can reach 45 to 50 miles per hour (75 to 80 km per hour) or more on steep slopes.
Source: U.S. National Park Service, Volcanic Processes: Lahars
Image: Hot lahar in the Nima II river near El Palmar, Guatemala, 1989, by USGS geologist Jeff Marso, public domain, via Wikimedia Commons.