I can not find any references to an underground river. There are no rivers here on the OS map from 1912, https://maps.nls.uk/view/114581590. The map references windmill but no water mills. The river is drawn as going under the ridge that mount road runs along, which suggests a lot of digging would have been required. Also the catchment area is presumable not very large.
So I suspect this underground river is a mistake and should be removed.