It rises south of Grantham close to South Witham at SK8818, passes through the centre of Grantham (where it may be closely followed using the Riverside Walk through Wyndham Park and Queen Elizabeth Park), passes Lincoln at SK9771 and at Boston, TF3244, flows into The Haven, a tidal arm of The Wash, near RSPB Frampton Marsh.

The Witham's course, which flows to the north and then to the south-east may be the result of glaciation (and possibly isostatic rebound) redirecting older rivers.

[citation needed] The source of the river is on high ground near South Witham, Lincolnshire,[6] at around 340 feet (100 m) above ordnance datum (AOD).

At Great Ponton, it is joined by the Cringle Brook on its left bank, and continues through Grantham, where it has already descended to 170 feet (52 m) AOD.

Beyond Claypole and near Barnby in the Willows it forms the border between Lincolnshire and Nottinghamshire for about 3 miles (4.8 km), before passing through an Army training area near Beckingham.

The River Brant joins on the right bank, before it passes through North Hykeham to reach Lincoln, where it is only 16 feet (4.9 m) AOD.

[8] In Lincoln, the river flows into Brayford Pool and exits along a narrow channel that passes under the medieval High Bridge.

[9] The bridge not only restricts navigation due to its small size, but the volume of water that can pass through the gap is limited in times of flood.

The path uses sections of the river towpath and abandoned railway tracks, and has been opened in stages, with the final 2 miles (3.2 km) being completed in September 2008.

The path is now part of Route 1 of the National Cycle Network and features a number of sculptures along its length, each commissioned from local artists.

However, it was known as the Grant Avon in Ancient British times, meaning divine stream, and the fact that the main town on the upper river was Grant-ham may support this.

Lincoln (Lindum)—the meeting point of Ermine Street, joining London to York, and Fosse Way, leading to Leicester and Bath—was an important Roman fort that became one of only four colonia in Britain.

Prior to the construction of the Grand Sluice, the lower river was affected by tides, but the highest they normally reached was Dogdyke or Chapel Hill, and Lincoln is considerably higher than these locations, by some 5 to 7 feet (1.5 to 2.1 m).

[21] Trading continued throughout the medieval period evidenced by the importance of Torksey, which was then a flourishing town, now only a small village.

The landowners then asked John Smeaton to liaise with Grundy and Edwards, and the three engineers produced a joint report in 1761, with estimates of £38,000 for drainage works and £7,400 for improvements to navigation.

[29] Once the Act of Parliament was obtained, Edwards became the engineer for the project, and drew up the detailed plans, which Grundy and Smeaton checked and altered slightly, after which they had no further involvement with the scheme.

He proposed two solutions; the first avoided the route through the city entirely, by utilising the course of the Sincil Dyke to the south, while the second involved lowering the bottom of the channel through the Glory Hole bridge, which was only 18 inches (46 cm) deep at normal water levels.

The Commissioners had imposed a toll on all traffic passing under the bridge, but decided that a channel bypassing the city would have grave financial consequences.

They opted for improving the existing channel[34] and the work to remove the wooden floor, to lower the river bed under the bridge and to underpin its foundations was completed in 1795.

[38] While Stamp End lock was being rebuilt, an alternative route was provided, utilising the Sincil Dyke and the South Delph.

However, the proprietors were still optimistic, and commissioned Sir John Rennie to investigate an extension to link the Witham to the River Ancholme, but although he made two proposals, neither was implemented.

[41] Most traffic was carried by sailing vessels or in barges hauled by horses, but in March 1816, the first steam packet boat arrived on the river.

This included the provision of fourth-class carriages, with fares set at a halfpenny per mile, in 1850, and by 1863, the steam packet boats had ceased operation.

Nevertheless, the railway company had to maintain the river, and in 1871, spent £5,000 on making Bardney lock deeper by 5 feet (1.5 m) at the request of the drainage commissioners.

Today, commercial traffic, apart from tour boats, has ceased above the port of Boston (The Haven) and only pleasure craft carry on through the lock at the Grand Sluice into the Witham.

[35] There are traffic lights on the short section between Brayford Pool and Stamp End Lock, which are used to indicate the state of the river in times of flood.

At Antons Gowt, a lock drops down into the Witham Navigable Drains, a system of drainage ditches which are used to prevent flooding of the fens to the north of Boston.

Since November 2008 there has been an active campaign by the Billinghay Skirth Regeneration Society to restore navigation on the River Skirth, and the project has won the support of Billinghay and other parish councils, the Inland Waterways Association, the Environment Agency, Lincolnshire County Council and the Lincolnshire Waterways Partnership.

Like most rivers in the UK, the chemical status changed from good to fail in 2019, due to the presence of polybrominated diphenyl ethers (PBDE), perfluorooctane sulphonate (PFOS) and mercury compounds, none of which had previously been included in the assessment.

[56] In March 2018, the river suffered the worst incidence of pollution ever recorded in Lincolnshire, when Omex Agriculture released ammonia into the water.