Stop locks Lock (water navigation)

lifford lane guillotine lock, kings norton, birmingham between stratford-upon-avon canal , worcester , birmingham canal

a stop lock (very) low-rise lock built @ junction of 2 (rival) canals prevent water passing between them.

during competitive years of english waterways system, established canal company refuse allow connection newer, adjacent one. situation created worcester bar in birmingham, goods had transshipped between boats on rival canals feet apart.

where junction built, either because older canal company saw advantage in connection, or new company managed insert mandatory connection act of parliament, old company seek protect (and enhance) water supply. normally, specify that, @ junction, newer canal must @ higher level existing canal. though drop newer older canal might few inches, difference in levels still required lock – called stop lock, because stop water flowing continuously between newer canal , older, lower one. lock under control of new company, , gates would, of course, point uphill - towards newer canal. protect water supply of newer canal, nevertheless donate lockful of water older company every time boat went through. in times of excess water, of course, lock bywash continuously supply water lower canal.

when variable conditions meant higher water level in new canal not guaranteed, older company build stop lock (under own control, gates pointing towards own canal) closed when new canal low. resulted in sequential pair of locks, gates pointing in opposite directions: 1 example @ hall green near kidsgrove, southern terminus of macclesfield canal joined hall green branch of earlier trent , mersey canal. 4 gate stop lock near kings norton junction, between stratford-upon-avon canal , worcester , birmingham canal replaced in 1914 pair of guillotine lock gates stopped water flow regardless of canal higher. these gates have been permanently open since nationalisation.

many stop locks removed or converted single gate after nationalisation in 1948. hall green stop lock remains, single lock: lock removed because lowering of t&m s summit pound (to improve harecastle tunnel s air draught – free height above water level) meant t&m lower macclesfield. hall green branch considered extension of macclesfield canal, meets t&m @ hardings wood junction (just short of harecastle tunnel north portal).

it should noted newer canal not @ higher level 1 joined. instance, there shallow lock @ autherley junction, 1835 birmingham , liverpool canal (now part of shropshire union canal) met older staffordshire , worcestershire canal, build in 1772. nicholson guide shows boater travelling south along newer canal locks before turning north or south onto older staffordshire , worcestershire canal – shropshire union canal gains small lockful of water each time boat passes. however, gain tiny since level difference small possible open both gates @ once.