for the slide. The villages of Incayoc and Huashau, closest to the base of Huascarán, were struck first. A group of Czechoslovak climbers on their way up the mountain were killed nearby. The entire Llanganuco delta area was destroyed, including the rebuilt town of Ranrahirca, where an estimated 1,800 people were killed. "An unknown, but probably large, number of additional casualties were sustained elsewhere in the extensive area covered by the debris lobe." As had occurred in Yungay, huge boulders were thrown hundreds of metres across the valley, destroying everything in their path. The largest single boulder found in the slide, near the site of Ranrahirca, weighed . The slide dammed the Río Santa, causing it to back up in a long lake. Material traveled as much as vertically up the opposite bank, destroying part of Matacoto village, before reversing and falling back into the river with a thunderous roar. Survivors described the motion as "like a wave on the shore." Within thirty minutes the landslide dam overflowed, and debris and mud flowed downstream for some at speeds up to , causing damage all along the river banks. In Caraz, downstream of Yungay, the airport and much of the surrounding farmland were flooded, and a highway bridge at Choquechaca was destroyed. The dam of the Cañon del Pato hydroelectric plant was also destroyed, although the power station itself was unharmed. At Huallanca below Cañon del Pato, the river reportedly dried up for an hour after the earthquake, due to its temporary blockage upstream, then abruptly rose from its bed. After the water receded "dozens of corpses, and vehicles and other manmade materials" were found deposited in the flood zone. Large sections of railroad and highway between Huallanca and Chimbote were either buried or washed away, cutting off access to the region. It reportedly took eight or nine days for the river to cut completely through the debris dam and restore its original course. Casualties and losses The number of deaths from the 1970 avalanche has been difficult to quantify; due to the sheer size of the devastated area and the depth to which the towns were buried, thousands of missing bodies were never found. Furthermore, the casualties in Yungay were greatly increased due to the disaster having occurred on Sunday – market day – when many people had come in from the countryside. Most estimates range between a total of