The Tangjiashan quake lake did not exist two weeks ago but the treacherous expanse now poses the biggest immediate threat to the already devastated earthquake-stricken area in Sichuan . Created when massive landslides poured into and blocked the meandering Fu River following the earthquake, Tangjiashan lake, hurriedly named after a nearby mountain, is 3.2km upstream from Beichuan county , the worst hit in the catastrophe. Just four in 10 people of the county's 20,000-plus population are thought to have survived the disaster. The water level in the lake has been rising in the past few days, creeping to within 29 metres of the lowest part of its embankment, Xinhua reports. Further down the Fu waterway lies Jiangyou county, where more than 20,000 residents had already been evacuated as part of an emergency plan to escape a potential flood in the event it bursts or flows over, said a high-ranking Ministry of Water Resources minister. 'And we have evacuation plans B and C, both on a bigger scale, in the pipeline,' Liu Ning, the ministry's chief engineer, said at a press conference in Beijing yesterday. 'The situation is grim, given the forecasted heavy rainfall in the next few days.' Meteorologists have predicted it will rain today and tomorrow, according to Xinhua. The lake, which already holds 100 million cubic metres of water, could swell drastically in a downpour. Officials skirted around questions about the number of people who would be affected and the fate of cities in the path of potential torrents. But a survey by the Chinese Academy of Sciences indicated that flooding 'could devour a city with a population of 1 million'. History also shows that authorities need to prepare for the worst. In 1933, a 7.5-maginitude quake decimated Mao county, only a few kilometres from Wenchuan , a town at the epicentre of the latest tremor, and triggered mudslides that blocked the Min River, a Yangtze tributary lying parallel to the Fu River in the west. The quake lake it formed overflowed 45 days later and drowned more than 20,000 people in nearby regions. In contrast, the quake itself killed about 7,000. Amid mounting concerns, E Jingping , a vice-minister of water resources, touched on the huge task at hand. 'The bottom line is that we are working to funnel the water away before it's too late,' he said. 'The problem is that poor weather has prevented us from air-dropping equipment and personnel that's needed to carve a diverting canal.' The lake became inaccessible to vehicles as all proper roads leading to it have been blocked by mudslides. Even if Mr E's men manage to keep things under control in Tangjiashan, it would hardly spell the end of troubles.