The Grand Union Canal was also the original name for part of what is now part of the Leicester Line of the modern Grand Union: this latter is now generally referred to as the Old Grand Union Canal where necessary to avoid ambiguity.
A 5-mile (8-km) section of the Oxford Canal forms the main line of the Grand Union between Braunston and Napton. Although the Grand Union intended to buy the Oxford Canal and Coventry Canal, this did not take place.
The section of the main line between Brentford and Braunston (formerly the Grand Junction Canal), was built as a 'wide' or 'broad' canal - that is, its locks were wide enough to accommodate two narrowboats abreast (side by side) or a single wide barge up to 14 feet (4.27 m) in beam.
However, the onward sections from Braunston to Birmingham had been built as 'narrow' canals – that is, the locks could accommodate only a single narrowboat. An Act of Parliament of 1931 was passed authorising a key part of the modernisation scheme of the Grand Union, supported by Government grants. The narrow locks (and several bridges) between Napton and Camp Hill Top Lock in Birmingham were rebuilt to take widebeam boats or barges up to 12 feet 6 inches (3.81 m) in beam, or two narrowboats. The canal was dredged and bank improvements carried out: the depth was increased to 5 feet 6 inches (1.68 m) to allow heavier cargoes, and the minimum width increased to 26 feet (7.92 m) to enable two boats of 12 feet 6 inches to pass. Lock works were completed in 1934 when the Duke of Kent opened the new broad locks at Hatton, and other improvements finished by 1937.
However, these improvements to depth and width were never carried out between Braunston and London, nor were the locks from the top of Camp Hill Locks in Birmingham widened.
The three sections between Norton junction and the River Trent (collectively known as the 'Leicester line') are mixed in size. From Norton to Foxton, the route is a narrow canal. From below Foxton to Leicester it is a wide canal. From Leicester to the Trent, the route is effectively the River Soar and the locks and bridges are wide. Another Act of 1931 authorised the widening of the locks at Watford and Foxton, but with Government grants for this section not forthcoming, the work was not carried out.
The Grand Union Canal was nationalised in 1948, control transferring to the British Transport Commission, and in 1962 to the British Waterways Board, later British Waterways. Commercial traffic continued to decline, effectively ceasing in the 1970s. However, leisure traffic took over, and the canal is now as busy as it ever was, with leisure boating complemented by fishing, towpath walking and gongoozling.
From the Thames Lock, the canal and the River Brent are one and the same, and the waterway is semi-tidal until the double Gauging Lock (lock 100) at Brentford is reached. Just upstream of the Gauging Lock was a large canal basin, now known as Brentford Lock, from which the canal continues to follow the course of the River Brent through two more locks. The river and canal part company at the base of the Hanwell flight of locks (92-97), before two more locks take the canal to Norwood Green. It then heads westward over level ground through Southall, Hayes and West Drayton until it reaches the valley of the River Colne where it swings northward to Cowley through Uxbridge.
Three miles (5 km) from Norwood on this long level is Bulls Bridge Junction, once the site of the Grand Union Canal Carrying Company's main dockyard. At Bulls Bridge, the Paddington Arm branches off to the north and runs 12 miles (19 km) to join the Regents Canal at Little Venice (see below). Just before Uxbridge is Cowley Peachey junction, where the Slough Arm branches off westward.
At Cowley, the canal begins to climb the valley of the River Colne following a north-westerly course. After Uxbridge, there are many disused gravel workings in the valley so the canal is surrounded by lakes as it passes Denham and Harefield before veering north-east to Rickmansworth.
From Rickmansworth, the canal follows the valley of the River Gade, a tributary of the Colne. After passing the site of Croxley paper mill, the canal skirts Watford through Cassiobury Park, passes under the M25 motorway and approaches Kings Langley. The canal here passes the site of the former Ovaltine factory, which was once supplied with raw materials by canal.
By now the locks are becoming more frequent as the climb into the Chiltern Hills steepens. The original four locks here were replaced in 1819 by five shallower ones to alleviate problems with water supply to the nearby paper mills. This realigned the canal to the south of its former course; the locks here are still referred to - without irony - as "The New 'Uns" by traditional boaters, and the term has been passed on to a new generation of canal users.
After Kings Langley and Apsley - the site of more former paper mills - the canal passes Hemel Hempstead and Boxmoor Common. Next come Bourne End with the well-known swingbridge at Winkwell, and Berkhamsted. The last few miles to Tring summit follows the course of the River Bulbourne. At Cowroast Lock the canal reaches the 3-mile (5-km) long summit level at Tring in the Chiltern hills, having risen through 54 locks since Brentford.
At the north-west end of the summit level is Bulbourne Works, where lock gates were manufactured until 2003 for the southern canal network. Half a mile (800 m) further on, the canal reaches the top of the Marsworth flight of seven locks, which begin the descent to the Vale of Aylesbury. The Wendover arm branches off westwards from the summit level under a bridge adjacent to Marsworth top lock and is currently navigable for just over a mile to moorings and a winding hole. There is a restoration project to extend it back to Wendover. A few hundred yards (metres) beyond the bottom lock of the flight, the Aylesbury arm branches off to the south west.
The Grand Union crosses the wide valley gradually, descending by interspersed locks past the villages of Cheddington, Horton and Slapton until it reaches Leighton Buzzard. Traditionally this section of the canal is called "Slapton Fields" or just "The Fields" by boaters.
A few miles further on it enters Milton Keynes at the outskirts of Bletchley at Fenny Stratford lock, which is unusual in lowering the level by only 12 inches (30 cm). The next stretch of 11 miles (18 km) on the level takes the canal through the new city, where there is a marina. (There is a plan (see below) to dig a new arm from here to the Great Ouse at Bedford). North of the centre, it traverses the modern New Bradwell Aqueduct, the first on the Grand Union in over 100 years. Leaving Milton Keynes at Wolverton, the canal runs on a high embankment before passing over the Great Ouse at Cosgrove "Iron Trunk" aqueduct.
After rising through Cosgrove lock, (and passing the start of the abandoned Buckingham Arm) another long level section brings the canal to the bottom of the Stoke Bruerne flight of seven locks. At the top of this flight is the Stoke Bruerne Canal Museum followed shortly by Blisworth tunnel, at 3056 yards (2794 m) one of the longest on the canal network.
Once clear of the tunnel, the canal passes Blisworth village and reaches Gayton junction where the Northampton arm branches off to the east. This arm has seventeen narrow locks as it descends to join the navigable River Nene (see below). The long level stretch continues past several villages including Heyford and Weedon Bec and is very rural in character.
At Whilton, the canal reaches the bottom of the Buckby flight of seven locks which raise it to Braunston summit although the village of that name is still 5 miles (8 km) distant. Beyond the top lock is Norton junction where the Leicester line (not strictly a branch) heads off north. A few miles further on the canal passes through the 2040-yard (1865-m) Braunston Tunnel, which pierces a low range of hills that are part of the Northamptonshire uplands.
The canal then drops down the Braunston flight of six locks until it reaches Braunston junction, just over 93 miles (150 km) from Brentford.
Shortly after Napton Junction, the Grand Union reaches three locks at Calcutt, which begin the descent to the Warwickshire River Avon. After a 3-mile (5-km) level, the canal descends into the valley of the River Leam by the Stockton flight of ten locks (often known as 'the Itchington ten'). Above the eighth lock down the flight, a short arm (now used as pleasure craft moorings) used to serve Southam cement works.
From the bottom of the locks, a 3-mile (5-km) level leads to the four Bascote locks. The top two form a 'riser' or staircase (see Canal lock). Six more interspersed locks lead to Radford, after which a 5-mile (8-km) level takes the canal through Leamington Spa to Warwick. Between these two towns, the canal crosses the River Avon on an aqueduct.
At Warwick, the canal rises by two locks to Budbrooke junction (formerly the junction with the then-independent Warwick and Birmingham canal). To the left is the restored Saltisford Canal Arm, a short stretch that used to run under the railway to the original canal basin complex and terminus of the Warwick and Birmingham Canal- basin sadly filled-in in the 1970s. The Canal used to serve the oldest gas works in the world, several unusual hexagonal buildings remain opposite Sainsburys. Saltisford Canal Trust After half a mile the mainline,reaches the bottom of the Hatton flight of 21 locks that lift the canal up out of the Avon valley. The first ten locks are spaced out but from the middle lock the flight is tightly spaced.
Three miles (5 km) from Hatton top lock the canal passes through Shrewley Tunnel, with its separate horse tunnel, and then passes Rowington village to Kingswood junction where a short spur connects with the Stratford-upon-Avon Canal. Another 3 miles (5 km) lead to the Knowle flight of five locks. Finally, an 11-mile (18-km) level takes the canal through Elmdon Heath, Solihull, Acocks Green, and Tyseley to the heart of Birmingham.
The main line may be considered to terminate at Bordesley Junction. From here, there are two routes, both part of the Grand Union Canal. The original line of the Warwick and Birmingham Canal leads to the Digbeth Branch Canal of the Birmingham Canal Navigations at the Warwick Bar, while the later line of the Birmingham and Warwick Junction Canal leads to the Birmingham and Fazeley Canal (and Tame Valley Canal) at Salford Junction, which in turn has connections to the Coventry Canal and the Trent and Mersey Canal.
Also on this section are the well-known features of Foxton Locks and Watford Locks, both of which feature staircase locks. Beside Foxton locks is the site of a long-abandoned inclined plane boat lift. This was constructed as part of a project to create a wide-beam canal route to connect the northern and southern parts of the canal system, something which does not exist to this day. Funding to deal with the narrow locks at Watford was not forthcoming and the scheme was aborted. From Foxton Junction, at the bottom of the locks, the canal is wide-beam to Leicester and onwards, as it was originally intended to link the Soar at Leicester with the River Nene at Northampton. However, the canal never went further than the basin at Market Harborough.
The Grand Union Canal has several branches, usually termed 'arms'. Five miles (8 km) from Brentford, the Paddington arm leads to Paddington Basin and, just north-west of the Basin at 'Little Venice', it connects to the Regent's Canal. At Cowley Peachey, the Slough arm runs 5 miles (8 km) to the west. At Marsworth, about 35 miles (56 km) from Brentford, two arms leave the main line, one to Wendover (not currently navigable for its full length but being restored by the Wendover Arm Trust) and the other descends through sixteen narrow locks for 4 miles (6 km) to Aylesbury. From Gayton junction, about 60 miles (97 km) from Brentford, the Northampton arm links with the River Nene.
At Warwick the last remaining branch of the GU, the Saltisford Canal Arm is encountered. The restored arm is close to the centre of Warwick, and is now a short branch of the Grand Union Canal, but was originally the mainline line of the Warwick and Birmingham Canal, 1799, leading to the terminus and a basin complex built to originally handle timber. When the Warwick and Napton Canal opened, this section was by passed but served as the town's wharf area. The Saltisford Canal Trust have restored most of the surviving canal over the last 25 years, including major piling works and restoration of a warehouse in 2007. The last 150 metres were lost in the 1970s and are now built on except for a bridge now isolated in a carpark. The restored section is now the mooring for colourful narrowboats and a waterside park open to the public. Over 800 visiting narrowboats come by water to Warwick each year and moor on the arm. Saltisford Canal Trust
On the Leicester Line there are two arms. One is a mile (1.6 km) long and leads to the village of Welford. The other leaves the main canal at the bottom of Foxton locks and runs 5 miles (8 km) to Market Harborough.