Volcanic Explosivity Index scale, the eruption was rated a 5, and categorized as a Plinian eruption. The collapse of the northern flank of St. Helens mixed with ice, snow, and water to create lahars (volcanic mudflows). The lahars flowed many miles down the Toutle and Cowlitz Rivers, destroying bridges and lumber camps. A total of of material was transported south into the Columbia River by the mudflows. For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 16 miles (20 to 27 km) above sea level. The plume moved eastward at an average speed of with ash reaching Idaho by noon. Ashes from the eruption were found on top of cars and roofs the next morning as far away as Edmonton, Alberta, Canada. By about 5:30 p.m. on May 18, the vertical ash column declined in stature, and less-severe outbursts continued through the night and for the next several days. The St. Helens May 18 eruption released 24 megatons of thermal energy and ejected more than of material. The removal of the north side of the mountain reduced St. Helens' height by about and left a crater to wide and deep, with its north end open in a huge breach. The eruption killed 57 people, nearly 7,000 big-game animals (deer, elk, and bear), and an estimated 12 million fish from a hatchery. It destroyed or extensively damaged more than 200 homes, of highway, and of railways. Between 1980 and 1986, activity continued at Mount St. Helens, with a new lava dome forming in the crater. Numerous small explosions and dome-building eruptions occurred. From December 7, 1989, to January 6, 1990, and from November 5, 1990, to February 14, 1991, the mountain erupted, sometimes huge clouds of ash. 2004 to 2008 activity Magma reached the surface of the volcano about October 11, 2004, resulting in the building of a new lava dome on the existing dome's south side. This new dome continued to grow throughout 2005 and into 2006. Several transient features were observed, such as a lava spine nicknamed the "whaleback", which comprised long shafts of solidified magma being extruded by the pressure of magma beneath. These features were fragile and broke down soon after they were formed. On July 2, 2005, the tip of the whaleback broke off, causing a rockfall that sent ash and dust several hundred meters into the air. Mount St.