River flow has the final say
On the Semois, the Walloon Region closes navigation below 2.2 m³/s, to protect fish and the riverbed, and above 50 m³/s, for safety reasons. Between the two, the feel of the river changes clearly:
- 2.2 to 10 m³/s: low water. A relaxed paddle, with a few stones to avoid.
- 10 to 30 m³/s: ideal water level. Good speed, maximum enjoyment.
- 30 to 50 m³/s: sporty level. Strong current.
The Semois water level page shows the live reading. For the thresholds on the other rivers, see the navigable flows page.
Month by month on the Semois
Spring, April to June
Flows are generous, the banks green and the water lively. This is when the water crowfoot is in flower. The water stays cool: best avoided with young children.
Summer, July and August
The water is shallower and warmer, which means more swimming stops. Weekends are very busy: book ahead and preferably set off in the morning. Late in summer, a flow that is too low can become a problem.
Autumn, September and October
Autumn colours and calm return to the water. The flow rises with the autumn rains: check it before you go.
Winter
Floods are frequent and the Lesse hire companies stop sending boats out. The Semois is not recommended at this time of year.
Permitted hours
Downstream of Herbeumont, where the Alle-sur-Semois routes run, navigation is allowed from 9:30 am to 7 pm until 15 June, then from 9:30 am to 8 pm from 16 June to 15 October. Plan your start accordingly: on the 17 km route, set off early so you do not miss the last shuttle.
Rivers that close in summer
Some waterways open only in winter, from 1 October to 15 March, from 10 am to 5 pm:
- the Salm, as long as the flow stays below 9.7 m³/s at Trois-Ponts;
- the stretch of the Lesse between Han-sur-Lesse and Houyet.
The Viroin, by contrast, is open all year within its flow limits, except on the two fishing-opening days. It often drops in July and August, so check its level before you book.
And on the Lesse?
In high season, the standard flow sits between 8 and 15 m³/s: the trip is smooth for a group. July and August weekends are very busy, with several hundred kayaks on the water. April, May, June and September offer a better balance between flow and crowds. Navigation is forbidden above 52 m³/s at Gendron, and when the 72-hour average falls below 1.5 m³/s. On the Semois, the low threshold is calculated the same way: a 72-hour average below 2.2 m³/s at Membre.
Choosing by priority
| You want | Recommended period |
|---|---|
| Warm water, swimming | July and August, preferably in the morning |
| Young children | June to September: milder air and water |
| A helpful current and greenery | April to June |
| Fewer people, autumn colours | September and October |
| Whitewater on a winter river | 1 October to 15 March, with a club |
For families, the family kayaking guide details the best window. To compare rivers, see which river to choose.