Information about the 29km long canalised river Aa


The canalised river Aa was like a ‘back door’ into French waterways from the port of Gravelines, until the tide sluices and movable bridges in the port were closed in 2017. The river is now navigable from just south of Gravelines to Saint-Omer, a distance of 27km. The 0.7km long ‘maritime waterway’ formerly managed by the Port of Gravelines is now closed since the authority was formally wound up in 2017. At the other end, the Aa formerly connected in Saint-Omer with the Canal de Neuffossé (or Canal d’Arques à Saint-Omer), but the waterways in the area were completely transformed when the high-capacity Dunkerque-Escaut waterway was built in the 1960s. Saint-Omer was bypassed, and part of the former through route was closed to navigation, with the result that access to the centre of Saint-Omer is from upstream (see Dunkerque-Escaut waterway).

Although no longer an entry port, Gravelines is a historic village port that contrasts with its bigger neighbours Calais and Dunkirk. The non-tidal marina basin dries partly, but does so to very soft mud of a depth not to compromise even fin keel yachts. The tidal sluice gates, one set on either side of the marina (see plan, below), are old and lo longer operated, as indicated.

Gravelines - river Aa location

The passage up the entrance channel to Gravelines is quite feasible and straightforward approximately 2 hours either side of HW, although this will depend on draught. It is not to be attempted in adverse conditions, including on-shore breezes more than force 4. There is 1km of concrete breakwater protected channel through the beaches, then another 3km to the marina entrance itself. Mast stepping or unstepping will benefit from some preparatory work (see under Dunkirk port and Canals, and in Practical Navigation Introduction). The chandlery at the marina can arrange craneage through a private firm.



History – Works to make this fenland river navigable were started in 1114 under the Count Baudouin VII of Flanders. They were completed in 1320 under Jean III. Navigation developed in this part of France before the invention of the pound lock, and made use of primitive inclined planes or overdrach. The Aa and the port of Gravelines were vital to commerce in Flanders from the mid-17th century, when Vauban drew up the first plans for the Canal de Neuffossé to link with Lille. Today it remains in the strategic national network despite its mixed identity: part of it belongs to the high-capacity waterway, while the lower section below the junction with the Canal de Calais is a tranquil recreational waterway which deserves to see more use. The River Houlle is also a delightful branch and well worth the detour. Restoration of the disused section to link with the old Canal de Neuffossé in Saint-Omer is currently envisaged by the local authority. 

Key Waterway Dimensions

  • Max Beam: 5.50m
  • Max Height: 4.20m
  • Max Draught: 1.80m


South from Gravelines, the Aa is a tranquil fenland river, connecting with the Canal de Calais and the Canal de Bourbourg. Then from PK 11, over a distance of 9km, the Aa is incorporated in the high-capacity waterway, and it is the PK of this waterway that are shown on the posts on the right bank. The route description is continued here throughout from Gravelines to Saint-Omer, ignoring these ‘official’ distances of the grand gabarit waterway. Within this section, an attractive lock-free branch, the river Houlle, extends 4km from the Aa to the village of Houlle.
The last 2.1km section of the Aa from Saint-Omer to the junction with the through route of the Dunkerque-Escaut waterway is closed to navigation. The lock at Haut-Pont has been out of use for years, and the whole section has silted up badly. It is to be hoped that the local authorities will recognise the value of the old waterway through Saint-Omer for tourism and find the means to restore this disused section.
From the junction at PK 2.1, over a distance of 8km, the Aa is incorporated in the high-capacity waterway, and it is the kilometre distances of this waterway that are shown on the posts on the right bank. For the sake of clarity, the route description is continued here throughout from Saint-Omer to Gravelines, with official distances on the left and actual distances on the right. (There is a difference of 0.3km due to the shortening of the original course by the works for the Dunkerque-Escaut waterway.) Further downstream, the Aa retains its identity as a tranquil fenland river, connecting with the Canal de Calais and the Canal de Bourbourg. An attractive lock-free branch, the river Houlle, extends 4km from the Aa at PK 118.1 to the village of Houlle.

Locks – The complex system of tide sluice gates in Gravelines, which ensures automatic discharge of the river flow to the sea and protection against high water, is no longer in operation for boats. Navigation is lock-free from Gravelines to Saint-Omer. The former lock (Haut-Pont) at Saint-Omer is out of use.

Draught The maximum authorised draught is 3.00m in the upgraded section (PK 2 to km 10.5), then 2.00m to the junction with the Canal de Calais, 1.80m from here down to Gravelines. The maximum draught on the Houlle is 1.20m.

Headroom Fixed bridges leave a minimum headroom of 4.20m above the highest navigable water level, increased to 5.25m in the improved section, and 5.40m between km 15.1 and Gravelines (respectively 4.45m, 5.55m and 5.65m above normal levels).

Towpath There is a good towpath throughout, at first along the right bank, changing to the left bank at Le Guindal.

Authority VNF Direction territoriale Nord – Pas de Calais, Lille.
–    Rue de l’Écluse Saint-Bertin, BP 353, 62505 Saint-Omer cedex (PK 0-9)
–    Terre-plein de l’Écluse du Jeu de Mail, BP 1008, 59375 Dunkerque (PK 9-27.5)

Click to enlarge


Route description

PK 29.1    Gravelines tide sluice gates, swing bridge, access to harbour and North Sea


Port de Plaisance: VHF #9 harbour control for the lock gates. The marina offers all services, good restaurant on the quayside and more shops restaurants and bars in the town.
The town itself is historic and because it lay on the border between France and the Spanish Netherlands shows a fortified ‘star’ plan, surrounded by a moat that can nowadays be rowed around. The Spanish Armada anchored off Gravelines in 1588, preparatory to taking on experienced fighting troops and launching an invasion of England, foiled by the (far smaller) Royal Navy’s action using fire-ships that effectively destroyed the Armada and sent it on a calamitous voyage around the British Isles.

PK 28.9    Bassin Vauban, yacht harbour, 30 visitors’ berths, night €22.20, crane 7t, slipway, pump-out, repairs, restaurant
N.B. No connection between the port and the river Aa
PK 28.7    Saint-Folquin tide sluice gates and fixed bridge
PK 28.6    Bridge
PK 28.5    Gravelines quay and boat moorings r/b, village 500m
PK 28.1    Railway lift bridge, industrial quays u/s
PK 27.5    Automatic lift bridge (D940/D601)
End of section no longer in operation, river navigable from this point inland
PK 24.7    Motorway bridge (A16)
PK 24.5    Junction with Canal de Mardyck l/b (small craft)
PK 23.6    Saint-Folquin, lift bridge, village 2000m l/b
PK 22.9    Junction with Canal de Bourbourg, r/b (Le Guindal)

The lift bridges between Gravelines and Le Guindal are opened by a travelling bridge-keeper, from Le Guindal onwards they are manned. The Canal de Bourbourg that enters the Aa here has its starting point at Dunkerque; a short detour along it to the pretty village of Bourbourg is worthwhile; there is a good pontoon.

PK 20.5    Lift bridge (Saint Nicolas)

The lift bridge at La Bistade, and a useful bollard. © Épaulard59
The lift bridge at La Bistade, and a useful bollard. © Épaulard59

PK 17.4    Lift bridge (Bistade), private quay d/s l/b
PK 15.1    Junction with Canal de Calais, l/b (Pont du West)
PK 11.6    Bridge (Ruth)
PK 11.3    LGV railway viaduct (line to Channel Tunnel)
PK 11.2    Pipeline crossing
PK 11.0    Road bridge (D600 Saint Omer-Dunkerque)
PK 10.7     End of section used by Dunkerque-Escaut waterway (PK 120.9), Aa branches off left
PK 9.6    Boat moorings (relais fluvial) on La Bombe, l/b
PK 9.5    Watten bridge, quay d/s r/b, navigation continues in Watten diversion canal l/b
PK 8.9    Port Lermitage former boat harbour r/b, slipway
PK 7.6    End of cutoff, junction with river Houlle l/b, 4.0km to the village of Houlle (see below)
PK 7.2    Cutoff (bypassing bend in river) r/b
PK 4.0    Saint-Momelin bridge, small village r/b
PK 3.8    Confluence of Moerlack, r/b (entrance to Wateringues canals navigable in small craft)

PK 2.1     Junction with liaison Dunkerque-Escaut (PK 112.6) r/b, beginning of section incorporated in Liaison Dunkerque-Escaut, possible mooring for Saint-Omer in basin r/b
PK 1.2    Lock (Haut Pont), disused, restoration under study in 2010
PK 0.0    Saint-Omer basin, former junction with Canal de Neuffossé, accessible only from u/s (branch of Liaison Dunkerque-Escaut)

River Houlle

The charming river Houlle. © Épaulard59
The charming river Houlle. © Épaulard59

PK 0.0    Lowestel bridge,  junction with liaison Dunkerque-Escaut  at PK 118.1, (Aa PK 7.6)
PK 0.1    Railway bridge
PK 0.4    Junction with river Muissens (small boats only), r/b
PK 0.5    Junction with river Reninghe (small boats only) l/b
PK 1.3    Café l/b, moorings
PK 1.9    Road bridge (D600)
PK 2.1    Disused basin (former brick factory) l/b
PK 3.3    Camp-site l/b
PK 3.5    Houlle moorings l/b, village with shops, café, restaurant 300m
PK 3.6    Former grain loading basin, r/b
PK 3.7    Camp site, r/b, mooring possible
PK 4.0    Moulin de Lafoscade, limit of navigation but inaccessible on account of silting