The flooding happened after the rivers Adour and Midouze - and their tributaries the Douze and Midou - burst their banks.
The waters hit a peak of 6.36 metres in the commune - and department capital - of Mont-de-Marsan, said flood alert service Vigicrues.
This is the highest level since 1981, said weather reporter Guillaume Woznica at news source LCI. Vigicrues said the level was higher than the previous peaks of 2009 (5.84 metres) and 2014 (5.41 metres).
#Inondations : les #Landes sont toujours en #VigilanceOrange ce matin, notamment pour les bassins de l'#Adour et de la #Midouze. Le pic de crue de la Midouze a été atteint hier soir à #MontDeMarsan avec une cote de 6,36 m, qui a connu sa crue la plus importante depuis 1981 ! @LCI pic.twitter.com/jeoHkzDqX2— Guillaume Woznica (@GWoznica) May 13, 2020
Forecaster Météo France has so far maintained an orange alert for Landes. This is the second-most severe alert level, meaning the situation is potentially dangerous.
Météo France warned residents to be alert and to not - under any circumstances - attempt to enter any flooded areas, whether on foot or in a car.
It added that water levels could rise rapidly without warning, and said residents should secure their belongings to avoid further damage.
In a statement, Vigicrues said: "Although the rain has now stopped, the floods around the Midouze are causing damaging overflows, particularly downstream of the section in the Mont-de-Marsan and Tartas. Around the Adour, the overflow will be significant.”
Yellow Météo France alerts are in place in 30 other departments.
Sign up to our free weekly e-newsletter
Subscribe to access all our online articles and receive our printed monthly newspaper The Connexion at your home. News analysis, features and practical help for English-speakers in France