Question: The 12th Boat Race took place on a river that is how long?

Context paragraphs:
The Boat Race 1922: The 74th Boat Race took place on 1 April 1922.  Held annually, the Boat Race is a side-by-side rowing race between crews from the Universities of Oxford and Cambridge along the River Thames.  Cambridge went into the race as reigning champions, having won the previous year's race, and had the significantly heavier crew.  Four of the Oxford crew and three of the Cambridge crew had previous Boat Race experience.  In this year's race, umpired by former rower Frederick I. Pitman, Cambridge won by four-and-a-half lengths in a time of 19 minutes 27 seconds.  It was Cambridge's fourth consecutive victory, the largest winning margin since 1914 and the fastest winning time since 1911.

The Boat Race 1927: The 79th Boat Race took place on 2 April 1927.  Held annually, the Boat Race is a side-by-side rowing race between crews from the Universities of Oxford and Cambridge along the River Thames.  Oxford's crew was marginally heavier than their opponents, and saw five participants return with Boat Race experience, compared to Cambridge's four.  Umpired for the first time by former Oxford rower Charles Burnell, Cambridge won by three lengths in a time of 20 minutes 14 seconds.  It was the first race in the history of the event to be broadcast live on BBC Radio.  The victory took the overall record in the event to 40–38 in Oxford's favour.  The inaugural Women's Boat Race was contested this year, with Oxford securing the victory.

The Boat Race 1938: The 90th Boat Race took place on 2 April 1938.  Held annually, the Boat Race is a side-by-side rowing race between crews from the Universities of Oxford and Cambridge along the River Thames.  It was the first Boat Race to be televised.  In a race umpired by the former Cambridge rower and coach Harold Rickett, Oxford won by two lengths in a time of 20 minutes 30 seconds.  The victory took the overall record in the event to 47–42 in Cambridge's favour.

The Boat Race 1891: The 48th Boat Race took place on 21 March 1891.  The Boat Race is an annual side-by-side rowing race between crews from the Universities of Oxford and Cambridge along the River Thames.  Oxford went into the race leading 24–22 in the event overall.  In total, eight rowers who were participating had previous Boat Race experience.  Umpired by former Oxford rower Frank Willan, pre-race favourites Oxford won by half-a-length in a time of 21 minutes 48 seconds.  It was Oxford's narrowest winning margin since the 1867 race.

The Boat Race 1854: The 12th Boat Race took place on the River Thames on 8 April 1854.  Typically held annually, the event is a side-by-side rowing race between crews from the Universities of Oxford and Cambridge.  The race was won by Oxford who triumphed over Cambridge by seven lengths.

River Thames: The River Thames ( ) is a river that flows through southern England, most notably through London.  At 215 mi , it is the longest river entirely in England and the second longest in the United Kingdom, after the River Severn.  It also flows through Oxford (where it is called Isis), Reading, Henley-on-Thames and Windsor.  The lower reaches of the river are called the Tideway, derived from its long tidal reach up to Teddington Lock.  It rises at Thames Head in Gloucestershire, and flows into the North Sea via the Thames Estuary.  The Thames drains the whole of Greater London.

The Boat Race 1992: The 138th Boat Race took place on 4 April 1992.  Held annually, the Boat Race is a side-by-side rowing race between crews from the Universities of Oxford and Cambridge along the River Thames.  Oxford's cox Andrew Probert was the oldest competitor in Boat Race history at the age of 38 years and 86 days.  Oxford won by one-and-a-quarter lengths, the closest margin of victory for twenty years.  The race also featured the first German competitor in the history of the event in Dirk Bangert.  Umpired by former Cambridge rower Roger Stephens, Mike Rosewell writing in "The Times" described the race as "one of the greatest races since 1829".

The Boat Race 2008: The 154th Boat Race took place on 29 March 2008.  Held annually, the event is a side-by-side rowing race between crews from the Universities of Oxford and Cambridge along the River Thames.  Oxford won the race.  Oxford's crew featured the oldest competitor in Boat Race history.  The race took place in very difficult weather conditions – strong winds and heavy rain – resulting in the slowest winning time in over sixty years.  Oxford won by six lengths, the largest margin of victory since the 2004 race.

The Boat Race 1965: The 111th Boat Race took place on 26 March 1966.  Held annually, the event is a side-by-side rowing race between crews from the Universities of Oxford and Cambridge along the River Thames.  The race was won by Oxford, who led from the start, by three-and-three-quarter-lengths, in the third fastest time in Boat Race history.  Isis won the inaugural reserve race while Cambridge won the Women's Boat Race.

The Boat Race 1936: The 88th Boat Race took place on 4 April 1936.  Held annually, the Boat Race is a side-by-side rowing race between crews from the Universities of Oxford and Cambridge along the River Thames.  Umpired by the former Boat Race coach Francis Escombe, Cambridge won by five lengths in a time of 21 minutes 6 seconds.  The record thirteenth consecutive victory took the overall record in the event to 47–40 in Cambridge's favour.  The heaviest crew up to that year in Boat Race history, Cambridge were the first to weigh more than an average of 13 st (82.4 kg) per individual.

Supporting paragraphs: The Boat Race 1854 River Thames
Answer: 215 mi