The pound lock holds water between two gates, and is considerably easier to navigate. Pound locks have been built in China since 983, in the Netherlands from 1065 and in Britain from the 1560s. Nevertheless, a few flash locks remained after the introduction of pound locks. Flash locks on the Nene continued to be used until they were replaced in a programme of modernisation, which included building new locks, carried out between 1936 and 1941. The last flash lock on the Thames was Hart's Lock, which lasted until 1937, while on the Lower Avon, the structure of Cropthorne Water Gate lasted until the reopening of the river to navigation in 1961, although it had not been used for navigation for many years before that. Although slightly different in concept, Thames Lock at Weybridge on the Wey Navigation had an additional single gate some below the lock, which when closed raises the water level above it, allowing boats which would normally foul the cill to work through the lock. It continued in use until 1969, to enable grain barges to use the lock, and although grain traffic stopped in that year, it recommenced in 1981, when the gate was again used. The gate is still used at the lock-keeper's discretion for boats which have a draught exceeding .