Wednesday's heavy rain meant the new flood plain at Centre Vale Park, Todmorden, was used for the first time.
Water is stored there when levels in the River Calder reach a certain height, reducing river levels through the town and helping prevent floo
ding downstream.
The park storage area holds 21,500 cubic metres of water, the approximate equivalent of nine Olympic swimming pools, and when river levels drop it empties automatically and discharges the water back into the river.
Only a fraction of the storage area's capacity was used on Wednesday - the first operation since the flood plain was constructed in 2007.
Steve Wragg, who was in charge of the Environment Agency's flooding control room on the day, said: "The Centre Vale Park flood storage area helps to reduce the risk of river flooding for 220 homes in Todmorden, and we're very pleased that it worked so well.
"But what happens in narrow steep-sided valleys like the Calder at Todmorden, is that rain comes down off the hills very quickly, sometimes in sheets. The local drainage systems are unable to cope due to the sheer volume of water and there can be problems for residents."
Environment Agency officers are on standby this weekend in case of more heavy rain in the area.
