By Panu Wongcha-um and Kay Johnson
BANGKOK (Reuters) - China has notified downstream neighbours it is holding back the Mekong River flow at a hydropower dam on the waterway's upper reaches for 20 days, as part of a new data-sharing pact, the Mekong River Commission (MRC) and Thailand said on Wednesday.
The statements came a day after a new U.S.-backed monitoring system said China had failed to notify downstream countries of water restrictions that started on Dec. 31.
China agreed last October to share water data with the MRC, an advisory body to Thailand, Laos, Cambodia and Vietnam that had long sought the information for planning.
More than 60 million people in those countries depend on the river for fishing and farming.
Thailand's National Water Command Center said China had on Tuesday notified Thailand that its Jinghong Dam will from Jan. 5 to 24 reduce the water discharge rate from 1,904 cubic metres per second to 1,000 cubic metres per second, which amounts to a decrease of about 47%.
That was for "maintenance of transmission lines" in its electricity grid, it said.
The MRC said it received notification the same day, though it first detected water levels dropping on Dec. 31. The level would likely drop by about 1.2 metres and river navigation and fishing could be affected, it said.
The MRC said the Chinese notification assured the flow "will be gradually restored to its normal operation status on January 25", without specifying an exact volume rate.
The new Mekong Dam Monitor on its Facebook page on Tuesday said China had not notified neighbors when the Jinghong dam starting restricting waters on Dec. 31, "causing a sudden 1 metre drop in river level" downstream that could devastate the fish population.
The monitor uses cloud-piercing satellites to track levels at 11 upstream dams in China and those in other countries. Partly-funded by the U.S. State Department, it started operations last month, adding to the China-U.S. superpower rivalry in Southeast Asia.
Chinese authorities could not immediately be reached for comment. Beijing has rejected suggestions its Mekong dams harm downstream countries.
(Reporting by Panu Wongcha-um and Kay Johnson; Editing by Martin Petty)