The reserve is on two tributaries of the River Great Ouse, the Old Bedford River and the New Bedford River (also known as Hundred Foot Drain). The rivers were built by the 4th Earl of Bedford to facilitate drainage of the Great Ouse between Earith and Downham Market, and the "washes" (the area between the two rivers) are completely flooded for an average of 22 days per year, necessitating a 30 mile detour for local residents. With the advent of global warming, the washed now experience summer flooding also, and many birds are prevented from breeding in the area.
The area is also the site of RSPB Ouse Washes; it is a Ramsar site, a Special Protection Area (SPA) and a Special Area of Conservation (SAC), as well as the Ouse Washes SSSI - a Site of Special Scientific Interest.