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

Context paragraphs:
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.

HMS Ark Royal (R07): HMS "Ark Royal" was a light aircraft carrier and former flagship of the Royal Navy.  She was the third and final vessel of "Invincible"-class.  She was built by Swan Hunter on the River Tyne and launched by them in 1981.  Ark Royal was named by Queen Elizabeth The Queen Mother.  She followed sister ships HMS "Invincible" and HMS "Illustrious" into service in 1985.

HMS Cochrane (1905): HMS "Cochrane" was a "Warrior"-class armoured cruiser built for the Royal Navy in the first decade of the 20th century.  She served in the 2nd Cruiser Squadron during the First World War under Rear-Admiral Herbert Heath, taking part in the Battle of Jutland in 1916.  She was based in Murmansk in mid-1918 during the Allied intervention in the Russian Civil War.  She became stranded in the River Mersey on 14 November 1918 and broke in two.  The wreck was broken up in place by June 1919.

HMS Mersey: Five ships and a shore establishment of the Royal Navy have been named HMS "Mersey" after the River Mersey:

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.

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refit in 1998, she was named MV "Overchurch", she began service for Birkenhead Corporation Ferries in 1962, and was withdrawn from regular service by Mersey Ferries in 1990 where she saw minimal active work until 1999, when she was extensively refurbished.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  "Royal Daffodil" was withdrawn from regular service in December 2012 and returned to the ferries berth at Duke St, Birkenhead.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.

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.

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.

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.

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.

Supporting paragraphs: HMS Mersey (P283) River Mersey
Answer: "boundary river"