Question: What is the translation of the river that the HMS "Mersey" is named after?

full_context: Document 1: MV Royal Daffodil: MV Royal Daffodil is a ferry based on the River Mersey, England. She is currently laid up due to cost cutting measures. From launch until a major refit in 1998, she was named MV "Overchurch", she began service for Birkenhead Corporation Ferries in 1962, and was withdrawn from regular service by Mersey Ferries in 1990 where she saw minimal active work until 1999, when she was extensively refurbished. Despite her extensive re-build, the ship still retains a similar profile. "Royal Daffodil" was withdrawn from regular service in December 2012 and returned to the ferries berth at Duke St, Birkenhead. The ship has remained idle since her dry docking and survey in January 2013 and is currently laid up.
Document 2: HMS Mersey: Five ships and a shore establishment of the Royal Navy have been named HMS "Mersey" after the River Mersey:
Document 3: HMS Rother (1904): HMS "Rother" was a Palmer Type River Class Destroyer ordered by the Royal Navy under the 1902 – 1903 Naval Estimates. Named after the River Rother a river East of Sheffield, South Yorkshire in England, she was the first ship to carry this name in the Royal Navy.
Document 4: River Mersey: The River Mersey is a river in the North West of England. Its name is derived from the Anglo-Saxon language and translates as "boundary river". The river may have been the border between the ancient kingdoms of Mercia and Northumbria and for centuries it formed part of the boundary between the historic counties of Lancashire and Cheshire.
Document 5: Cheshire: Cheshire ( or ; archaically the County Palatine of Chester) is a county in North West England, bordering Merseyside and Greater Manchester to the north, Derbyshire to the east, Staffordshire and Shropshire to the south and Wales to the west. Cheshire's county town is Chester; the largest town is Warrington.
Document 6: WD Mersey: The TSHD "WD Mersey" is a British trailing suction hopper dredger, owned and operated by Boskalis Westminster Ltd (known as Westminster Dredging Company until 2014) and built in Italy. The ship maintained ports around the British Isles, including Liverpool, the Manchester Ship Canal, Harwich, Heysham and the Firth of Clyde
Document 7: River Dee, Wales: The River Dee (Welsh: "Afon Dyfrdwy" , Latin: "Deva Fluvius" ) is a river in the United Kingdom. It flows through parts of both Wales and England, forming part of the border between the two countries.
Document 8: Humber-class monitor: The "Humber"-class monitors were three large gunboats under construction for the Brazilian Navy in Britain in 1913. Designed for service on the Amazon River, the ships were of shallow draft and heavy armament and were ideally suited to inshore, riverine and coastal work but unsuitable for service at sea, where their weight and light draft reduced their speed from a projected twelve knots to under four. The class comprised "Humber" , "Mersey" and "Severn" . All three were taken over by the Royal Navy shortly before the outbreak of the First World War and were commissioned as small monitors. All three saw extensive service during the war and were sold in 1919.
Document 9: Middlesex: Middlesex ( , abbreviation: Middx) is a historic county in south-east England. It is now entirely within the wider urbanised area of London. Its area is now also mostly within the ceremonial county of Greater London, with small sections in other neighbouring ceremonial counties. It was established in the Anglo-Saxon system from the territory of the Middle Saxons, and existed as an official unit until 1965. The historic county includes land stretching north of the River Thames from 3 mi east to 17 mi west of the City of London with the rivers Colne and Lea and a ridge of hills as the other boundaries. The largely low-lying county, dominated by clay in its north and alluvium on gravel in its south, was the second smallest county by area in 1831.
Document 10: MV Royal Iris of the Mersey: The MV "Royal Iris of the Mersey is a Mersey Ferry in operation on the River Mersey, England. From launch in Devon in 1959 until a major refurbishment in 2001, she was named MV "Mountwood.
Document 11: Mersey River (Nova Scotia): The Mersey River, formerly known as Rivière Rossignol by the Acadians, is a river in Nova Scotia, Canada. It is named after the River Mersey in Liverpool, England. The river proper flows from the eastern end of Eleven Mile Lake in Annapolis County southward to Kejimkujik Lake in Kejimkujik National Park, then through Lake Rossignol to empty into the Atlantic Ocean at the town of Liverpool, Nova Scotia. The true source of the river however is as far northwest as Sandy Bottom Lake (Annapolis County) or Tuskopeake Brook (Annapolis County) on the northern tributary.
Document 12: HMS Mersey (P283): The fifth and current HMS "Mersey" is a River-class offshore patrol vessel of the British Royal Navy. Named after the River Mersey, the ship is the first to be named Mersey in 84 years. Various tenders (predominantly Ton Class minesweepers) were renamed Mersey during their service with Mersey Division Royal Naval Reserve (HMS Eaglet) between the early 1950s and late 1970s.
Document 13: Kingdom of Northumbria: The Kingdom of Northumbria ( ; Old English: "Norþhymbra rīce" , "kingdom of the Northumbrians") was a medieval Anglian kingdom in what is now northern England and south-east Scotland, which subsequently became an earldom in a unified English kingdom. The name reflects the approximate southern limit to the kingdom's territory, the Humber estuary.
Document 14: HMS Trent (1877): HMS "Trent was a "Medina"-class gunboat launched in 1877. She was the fifth ship of the Royal Navy to be named after the River Trent. She was renamed HMS "Pembroke in 1905, and served off the coast of Tanganyika in 1915. She was renamed HMS "Gannet" in 1917 while serving as a diving tender. She was scrapped in 1923.
Document 15: HMS Euphrates (1866): HMS "Euphrates" was an iron-hulled troopship of the "Euphrates" class. She was designed for the transport of British troops to India, and launched in the River Mersey on 24 November 1866 by Laird Brothers of Birkenhead. She was the fourth and last Royal Navy ship to bear the name.
ground_truth: "boundary river"