Welches Dam is a hamlet and parish in Cambridgeshire around five miles northwest of Ely. It is to be found deep in the Great Fen beside the Old Bedford River and the parallel channel of the River Delph. The Forty Foot Drain enters the Old Bedford at Welches Dam. The nearest bridges across the Hundred Foot Washes are those at Mepal to the southwest and Welney to the northeast.
The parish covers an area of 2,355 acres of the fen, and within its bounds are Welches Dam itself, and Purls Bridge.
The history of the parish is tied to that of Great Fen and the battle to drain it in the first half of the 17th century. The original principal drainage channel for the area, the Old Bedford River, runs through the parish.
The hamlet is not ancient nor is its name; it is named after Edward Welsh who built a dam across the Bedford River. Welsh was an employee of the Adventurers, the 17th century entrepreneurs who invested in the results of Cornelius Vermuyden's great work to drain the fens.
Fortrey's Hall, the 17th century ancestral seat of the Fortrey family lies in the parish.
In 1849, the area became infamous after 11 of the 187 residents of the parish died in an outbreak of cholera.
The mission church of St Eanswythe was built in 1909.