Delta ID    # 23.

Drainage basin: Africa.

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Contributed  by Professor James Coleman, LSU. From: Coleman and Huh, 2004.

The Mangoky River rises in the central Highlands of Madagascar and enters the Indian Ocean in the Mozambique Channel. The eastern one-half of the basin drains Pre-cambrian sediments while the central and western basin drains a prominent series of north-south oriented Mesozoic fold belts. The basin area is 58,155 sq km in area and drainage density is relatively low (Figure 61). From its headwaters to the delta mouth, the main channel has a length of 570 km. Average elevation in the drainage basin is 778 m, with a maximum of 1,440 m and a minimum of 240 m. Most of the higher relief areas are located in the eastern-most portion of the basin. Average annual rainfall is 831 mm, with a maximum of 1,882 mm and a minimum of 240 mm. The rainy months occur from November through March and the average annual monthly rainfall during this period is 130 mm, while the average monthly rainfall during the dry season is only 19 mm.

23-i02. Island in the meander belt of the Mangoky River.

The channel in the well-defined alluvial valley displays a braided pattern, with numerous mid-channel islands. The average annual river discharge is 526 m3/sec with a maximum of 1,621 m3/sec, which occurs in January and a minimum of 93 m3/sec which occurs in October. Thus the discharge pattern is displays extremely erratic discharge characteristics. Examination of satellite images of differing dates indicates that changes in channel pattern is quite common in the alluvial valley as well as in the delta plain. The climate in the alluvial valley is quite dry and exposed sandy islands dot the main river course.

23-i06: Change in water-covered areas between 1985 and 2000, Mangoky Delta.

The delta displays a fan-shaped pattern and has two main distributaries. The area of the delta is 1,547 sq km. Braiding is prevalent in the distributary channels and it is obvious that the distributaries change course frequently as numerous abandoned channel courses can be seen on the satellite images. The abandoned delta is some four times larger that the active delta plain. Most of the delta plain is devoid of vegetation because of the harsh arid climate; salt pans and barren algal flats are common throughout the delta plain. In the area of the abandoned delta plain to the north, tidal channels are prominent and mangrove vegetation dominate the lower delta region [dark red color in 23-i03].

23-i03. Land use in 1985, Mangoky Delta.

23-i04. Land use in 2000, Mangoky Delta.

Wave energy is quite high and beach ridges front the entire delta plain, while stranded beaches can be seen in the delta plain, especially in the southern part of the delta [23-i01].

23-i01: Satellite image of the Mangoky Delta.

The barrier islands are relatively unstable and are constantly changing their geometry. The barriers average 3 10 km in length (Stutz and Pilkey, 2002) and occur as new islands that form at active river mouths that grow laterally through the accretion of recurved spits. Along the inactive delta, the barriers are exceedingly narrow and show evidence of active alongshore migration.

Geo-registering the 1985 and 2000 satellite images and importing them into ArcView allowed interpretation of various changes between the two dates and to calculate the changes. The delta plain in the satellite image shown on Plate 22b is 1,449 sq km (358,056 acres) in extent. Plate 22e shows the comparisons of the 1985 and 2000 images that were analyzed for changes in open water and for new land gain for the fifteen year period between the images.

23-i05:  Land use changes in the Mangoky Delta 1985-2000.

Within the delta plain, in 1985, there were 30,631 acres of open water, while in the 2000 image, there was 34,792 acres of open water. Thus some 4,161 acres of deltaic plain wetlands (red area in 23-i05) were converted the open water in the 15 year period. This represents nearly a two percent loss of wetlands. Most of the new open water resulted from shoreline erosion and changes in the channel pattern of the river course. New land was gained at the southernmost mouth of the active river mouth and from abandonment of the former river courses in the eastern delta plain. Some 14,751 acres of new land was gained in the 15 year period. Thus there was a net gain of wetlands and channel bottoms of 10,590 acres during the fifteen year period. Plate 22c compares changes in modification of the delta plain by man-induced changes (agriculture, industrial, etc.). The green colored region in the in Plate 22c was land under usage as of 1985. A total of 32,125 acres of the original delta plain had been reclaimed by 1985. Some fifteen years later (2000), a total of 54,383 acres of delta plain was being utilized by man, a loss of wetlands of 22,258 acres in the fifteen year period. Thus, between natural change and man-induced change, a net total change of 7,504 acres (or 2 percent loss) of former delta plain had been converted to either open water or man-induced land use. This represents an average annual rate of 500 acres/year.