Towards the northern end of the washes, the New Bedford River is joined by the River Delph at Welmore Lake Sluice. The origins of this channel, which is sometimes called the thirty-foot drain, are obscure, although it was built to assist the removal of water from the washlands, according to C. N. Cole writing in 1784. Prior to 1825, there was a dam where the sluice is located, and water was let out of the washland by cutting a slot in it, which the flood water enlarged as it passed through. The cost of rebuilding the dam at the end of the process was considerable, and this unsatisfactory arrangement was replaced by a sluice. At the same time, the New Bedford River was made wider and deeper, while some of the flood banks were also raised. At its southern end, the River Delph is connected to the Old Bedford River at a double bend near Welches Dam. The northern half of the Old Bedford River is no longer connected to its southern half. It is not known exactly when this configuration was built, although it may have been between 1789 and 1821, since Cole's map of 1789 shows the Old Bedford River proceeding in a straight line, and Baker's map of 1821 shows two rivers, each with a double bend. Blair argues for a date soon after the New Bedford River was built, however, since Welches Dam is named after Edmund Welche, who worked with Vermuyden, and the dam in question was originally built across the Old Bedford River.
