1) Weirs were initially reinforcements to an existing river bank, formed of piles, but the need to heighten and strengthen the defences resulted in the building of stone walls, often but not always linked to bridges. The verb 'to weir’ was used in such contexts.
1718 a cutt 4 yds broad to be wear’d on each side, Hampsthwaite
1776 for wairing and walling, Boothhouse Woods, Holmfirth
1839 It is absolutely necessary that the banks should be stayed by a weir wall, Golcar.