The job of constructing the tunnel in Southampton was given to Thomas Jenkins, but the work did not go well, and John Rennie was asked to inspect it.

Parts of the Salisbury arm soon became dry, and others became a small stream, but the section in Southampton became a stagnant ditch.

[4] Raising money and getting the legal powers, through the Marsh Improvement Act 1844, to fill canal in delayed work until 1846 and the final grassing over didn't take place until 1851.

A new, larger, railway tunnel was excavated on a different alignment and at a slightly higher level which resulted in the partial removal of the canal bore.

[8] The branch from the Test started close to God's House Tower, which at the time was on the riverbank; this area has changed a great deal, with reclamation of land for the ferry terminal.

The Itchen branch curved to the north-east to reach a lock in the vicinity of the wharf which was downstream of Northam Bridge.

Construction of the Southampton and Dorchester Railway in 1846 and of a gasworks in 1848 destroyed much of the route, and excavations along this length in the 1960s failed to find any trace of it.

[13] Several sections sank or collapsed during construction due to geological conditions, poor materials or improper supervision of the works.

[16] After repairs were completed and a formal inspection had taken place, a further 60-yard section began to sink and opening was delayed for two months whilst this was also dealt with.

The crossing of the new tunnel in no way affected this drainage...so the soil (a black Clay) continued firm enough to support the brickwork laid upon it.

But by the filling up solid of a portion of it, leaving a hollow interval...the accumulation of water in seeking an egress has entered into, saturated, and sodden the clay on which the new Tunnel stands, and it is now incapable of supporting its weight.

Further ground movements in 1975 led the city council to dig shafts at the former King Edward's School building on Havelock Road, and to drill boreholes into the remaining sections of the canal tunnel, to investigate the situation.

At this time, the former western tunnel portal lay beneath the then R J Mitchell museum to the north of Kingsbridge Lane.

Early maps of this area show a water feature in the grounds of Kingsfield, and later Kingsbridge House and the school on the site.

A line drawn between this and an earthwork feature shown on the 1846 map in what later became Palmerston Park approximates to the documented length of the canal tunnel of between 560 or 580 yards.

The next section of the canal lies beneath the northern part of the Southampton Central railway station site, but there is a small area of woodland to the north of the railway line about 400 yards (370 m) to the west of the station, where remains of the canal bank were still discernable in 1966.

In 1964, when the Tanners Brook flood relief scheme was being constructed, a six-foot-wide brick-arched culvert which carried the brook under the canal was unearthed by the Hampshire Rivers Board, the bed of this section of the canal being recorded as lying above the nearby high water mark and fed from the local streams it crossed.

Some 1,150 yards (1,050 m) to the east of Redbridge station, the route crossed to the south of the railway line, to run along the bank of the Test.

[24] The route of the canal here was marked on the 1871 Ordnance Survey map, and much of it still appeared on the 1961–1968 edition, some 90 years later.

The small section that remained between the two was pumped out by the Southampton Auxiliary Fire Service in 1966 so that it could be measured and recorded.

It followed the southern bank of the Dun fairly closely, and the associated earthworks can be readily seen on modern maps almost to East Dean.

A second reservoir was located to the south of the road,[32] and although no longer there, its outline broadly follows a gap in the tree cover on the modern map.