About 1,000 cargo vessels have been trapped on China's key north-south water channel after falling water levels blocked their passage into the Qiantang River.

The ships were stranded at the Sanbao lock, in the Beijing-Hangzhou Grand Canal, before it joins the Qiantang River in Zhejiang Province.

Zhejiang provincial maritime bureau has restricted large vessels entering the Qiantang through the lock since May 4, when the water level in the river dropped to 5.5 meters.

On Monday, it temporarily prohibited all vessels from entering the Hangzhou Section of the canal or Qiantang River.

The jam was the most serious in history, and big vessels were expected to pass through the lock when the water level in the Qiantang rose to about 7 meters, said Zhou Guangming, a maritime officer of Hangzhou, capital of Zhejiang.

The river would brace for a flood tide in a week, raising the possibility for the reopening of the lock, said Zhou.

The drop in water levels was caused by a rapid flow after this year's flooding season and changes to the river's silt content, said Zhou.