The area between the two Bedford Rivers, long and wide at their greatest separation near Welney, is below the rivers and below sea level, and is frequently flooded during winter. There is a sluice at Earith which is used to control the waters of the River Great Ouse. It was built with nine openings in 1637, but was subsequently rebuilt with seven openings in 1824, and again with three openings in 1954. When flows in the upper river are normal, the sluice gates are closed, and most of the flow is routed along the tidal New Bedford River. As flows increase, the sluices are opened, and water flows along the Old Bedford River, which progressively floods the of washlands which comprise the Ouse Washes. At Welney, these washes provide an ideal habitat for wildfowl and other birds, and the Wildfowl and Wetlands Trust has established a nature reserve here, the WWT Welney. The reserve occupies , and is accessible from the A1101 bridge by following a minor road to the carpark, and then crossing the New Bedford River by a footbridge. It is important for its wild flowers, dragonflies and butterflies, as well as the birds, which are most numerous when the washland is flooded.