Between these low ranges of hills the Ancholme river runs south to north through its flat, low–lying flood plain, with a north–south height difference of only a few metres. The town sits on alluvial soils of the Ancholme, and the area surrounding the town was previously a semi–flooded marsh known as carrs. A series of drainage improvements from the 1630s to the 1820s transformed the whole of the valley into arable land. The largest of the drainage channels is also a canal known as the New River Ancholme. The original course of the river has been obliterated in places by the drainage works, but its discontinuous surviving length is known as the Old River Ancholme.