The lowest bridging point is the location on a river which is crossed by a bridge at its closest point to the sea.
Historically - that is, before the development of engineering technology that allowed the construction of tunnels and high-level road bridges - the lowest bridging point of a river was frequently the point at which an important town or city grew up, and particularly where trade and commerce took place. The place could be served by roads on either side of the river, allowing access from a wide hinterland; had river transport available upstream; and often was at a location that allowed seagoing traffic to approach it from a downstream direction.
Examples of lowest bridging points in Britain include London, the lowest bridging point on the Thames; Lancaster, the lowest bridging point on the Lune; York, the lowest bridging point on the Ouse; and Gloucester, the lowest bridging point on the Severn.