Analysis of the model for a 4.40 m flood at the Mopti scale With a maximum water height of 4.40 m at the Mopti scale, the 1984 flood was the lowest recorded since 1922 , the onset of records. This poor flood resulted from very poor rainfall in the upstream basin of the Niger and Bani rivers, in humid and sub- humid bioclimates. Meanwhile in the Sahel it was the most severe drought recorded since the beginning of the 20 th century. , In Ké Macina, when the Niger River enters the Delta, the flood was higher by 0.54 m than expected from the propagation model proposed by J.P. Lamagat for a flood height of 4.40 m at the Mopti gauge, and this height was reached twice (3.96 m and 3.97 m) six weeks apart. If the flood of the Bani River observed at Sofara corresponds to the flood calculated by the model, the flood at the exit of the Débo at Akka shows a very strong deficit with a very short duration for the propagation along the Mopti to Akka reach (11 days versus 39 days). There is therefore a strong imbalance between the flood on the western branch of the Delta along the Diaka, which was well supplied, and the Eastern branch, the Bani and Niger towards Mopti, poorly supplied. A series of Landsat images (25 Oct, 10 Nov, 26 Nov) was available to analyze this exceptional year. The image of November 10 gives the best information on the extent of open water and the green vegetation across the Delta. The flooded areas calculated by the model (map Niv_440) extended on fragmented basins with a total area of open water and green vegetation of 423,299 ha. The transposition of the model in Veg7 - shape (Niv_440_Veg7) -, after correction of the residuals linked to the smoothing of the Veg7k3 matrix, gives a floodable area of 391,484 ha, the figure retained for comparison with the Landsat images. The flooded area appears fragmented, with a block around the Débo-Walado, poorly flooded central basins, and very little flood in southern basins such as Yongari-Mangari, Sebera, Bani and Pondori. The analysis of the Landsat images makes it possible to separate open water, the dense green vegetation of the flooded plains and the light green vegetation of the non-flooded plains and “ togge ” as well as bare soils. Overlaid on Veg7 (shape Veg7_1984), the flooded areas with or without dense vegetation where further separated as fully or partly flooded areas. The areas were calculated in two different ways: 1 . by counting pixels on Landsat 2 . by reporting Landsat data on shape Veg 7, resulting in Veg7_1984 If we consider that the partly flooded areas are effectively flooded at 50%, the flooded area assessed from the vegetation map in 1984 would be 321,225 ha, close to the 325,453 ha measured by pixel count on Landsat images. The relatively higher flood of the Niger River at Ké Macina is part of the reason for the mismatch in the distribution of the flood between the western and eastern parts of the Delta, but probably not the only contributing factor: the peculiar longitudinal altitude profile of the Diaka could also be a factor. Indeed, while the bottom slopes of the Bani and Niger rivers are quite regular (see graphs:profil_fond), this is not true of the Diaka (see CARIMA : a Mathematical model of the Niger River - SOGREAH, 1984). The Niger River has a bottom slope of 4.5 cm / km along 200 km before it decreases to 2 cm / km, 50 km upstream after it enters the Débo. The Bani river alternates reaches with slopes of 4.5 cm / km and reaches with very low slopes (between 1 cm and 0.8 cm per km). However, neither of them presents back slopes as the Diaka does, in particular between km 90 and 126 downstream from Diafarabé. These back slopes would explain the great extent of the Toggere Koumbe basin and account in part for the dysfunction of the model with equal flood depth isolines during poor floods such as that of 1984. The peculiarity of the altitude profile of the Diaka, linked to the transversal dune (NW-SE) extending along the stretch, Saré Béré-Sildé-Rogonta-Sormé. This specificity is not found east of the basin where the bottom slope of the Mayo Dembé reaches 4.3 cm / km before it zeroes 20 km before it joins the Débo, highlighting the difference between the better flooded northwest part of the Delta, and the poorly flooded south and east parts of the Delta.