Chinese survey maps a 20-kilometre debris flow that hit Gyirong in seven minutes

China's Ministry of Natural Resources released three-dimensional before-and-after imagery on Saturday that traces the mudslide which erased Gyirong Port on the Tibet-Nepal border.

Guo Zhaocheng, a senior engineer at the China Aero Geophysical Survey and Remote Sensing Center, said the source was a high glacier on the upper Cuoqian River in Nepal, at about 5,140 metres. Gyirong Port sits near 1,800 metres. The drop is about 3,300 metres over roughly 20 kilometres.

The timing is the part that matters for anyone still arguing about warning time. Guo put the collapse at about 10:52 a.m. on 26 August. Video at the port shows the front arriving at about 10:59 a.m. That is seven minutes for 20 kilometres, an average near 50 metres a second. A tourist camera in Gyirong County caught the high collapse by accident. Officials now treat that clip as the start of the clock.

The imagery shows buildings, roads and bridges that exist in the pre-disaster frame and cannot be picked out after it. Chinese rescue teams who reached the crossing on Friday described ruins. Gyirong handled about $600 million a year in Nepal-China trade and was built up as a Belt and Road gate. Satellite views after the pulse show a scoured valley floor where the compound stood.

Tibet's emergency management department, as of 1 a.m. Saturday, listed seven dead and 554 missing on the Chinese side, with two local villagers rescued. Engineering crews said they had cleared 2.5 kilometres of the damaged G216 highway in Gyirong County. Nepal's consul general in Lhasa, Laxmi Prasad Niraula, said pilgrims heading for Mount Kailash who had come through the port were being housed with help from the consulate and the regional government.

China says the source lay in Nepal, not on its side of the line. That statement is about liability as much as geology. The catchment sits on the north shoulder of the Langtang group. Once the ice and rock were in the channel, the river did not respect the border post.

A barrier lake at the meeting of the Chhochen Khola and Purepu Tsangpo drew a Level-IV alert from China earlier in the week. Officials later said the near-term risk had eased. The alert is the reason rescue work on the Nepalese Trishuli was paused when rain lifted the river again.

Seven minutes is not a civil-defence interval. Sirens and phone towers cannot empty a border market in that time. What the new imagery does offer is a mapped path: source elevation, channel, run-out and the missing buildings. That map is now the baseline for any second-pulse forecast and for any argument about who should have seen the glacier as a live threat before 10:52 a.m. on Wednesday.