WUHAN, May 29 (Xinhua) -- China's transport authorities have ordered the dredging of navigation channels in the middle and lower reaches of the Yangtze River to ensure the safety of shipping as the water level falls to the lowest point in two decades.

    A large silt dredgers has been withdrawn from a north China offshore construction site for dredging the channels at Heishazhou (Blacksand Shoal), 40 km upstream from Wuhu Bridge on the lower reaches, said Wan Dabin, a senior official with the Yangtze River Waterway Bureau affiliated to the Ministry of Communications.

    The country's longest river usually reaches its summer flood season in middle or late May, but this year the levels are abnormally low.

    "Though it's already the flood season, the water level in the river has kept dropping, which has hindered navigation," said Wan.

    By Monday, the water level monitored near Hankou on the middle reaches was 4.53 meters, two meters lower than the same period a year ago, while the water level stood at 1.97 meters near Wuhu on the lower reaches, more than four meters lower than the record attained a year ago.

    Less rain in the entire Yangtze River drainage area has been blamed as a key cause of the falling water levels.

    On May 1, the Ministry of Communications raised the safe navigation depth in the 1,891-km section between Chongqing on the upper reaches of the Yangtze and Wuhu on the lower reaches to 7.5 meters this summer, comparing with five meters in previous years.

    Monitoring of the navigable course near Wuhu, in Anhui Province, shows that within a section extending 1,200 meters around Heisha, the depth is just 7.05 meters, 45 cm below the safe level for this summer.

    Dredging near Heisha is deemed necessary to ensure a safe navigation because it is unlikely to rain in the foreseeable future.

    Wan said workers had intensified patrols on the middle and lower reaches to warn of drops in the water depth in certain navigable sections.

    "Eight more dredgers are standing by to help dredge the river wherever necessary," said Wan.