materials, which is in contrast with the dark colour 
in near-infrared images of silt, loam and clay with 
their high moisture content. 
1.2.3 Old river floodplain deposits 
On the band-5 images of all three years some areas 
in the river floodplain are separable with their 
sharply cut edges, smooth surfaces and different 
types of vegetation cover. They do not show either 
meander or embroidery landscape patterns. They might 
be older and, situated at a higher level above the 
floodplain, have not been effected by recent flood 
ing of rivers. These areas are possible remains of 
old terraces build up by silt, sand and gravel. 
1.2.4 Sand accumulations 
A few small areas, in and along the Ganges riverbed, 
can be directly recognized with their white colours 
on band-5 and band-7 images of all three years. Both 
the very high reflection of these sediments in the 
visible and the near-infrared and their location are 
typical indications of thick and coarse sandy accu 
mulations. It also indicates that new alluvium is 
still being deposited in the Ganges riverbed. 
1.2.5 Minor river floodplain deposits 
In the West of the study area downstream floodplain 
of a smaller river, the Madhumati river, extend into 
composite and partly tidal floodplains. Abandoned 
channels, lops of cut meanders, small ridges and 
narrow basins can be seen on the 1975 images. Slow 
running minor rivers such as this can deposit only 
finer materials (silt, loam, clay) on their 
downstream floodplain, and clay in lower basins. 
Rainwater flooding can have further contributed to 
clay deposition in the basins. More silty materials 
may have build up the ridges. The clay causes a 
reduction of the near-infrared reflectivity of the 
clay-silt combination of basins which appear medium- 
grey on the 1973 and 1975 band-7 images. In the 
visible part of the spectrum, however, silts 
dominating the reflectivity of a clay-silt combina 
tion cause basins to appear light-grey on band-5 
1973 and 1975 images. 
1.3 Subdivision of the tidal floodplain into 
lithologic units 
In the tidal floodplain high tide water flowing back 
from the land into rivers during low tide cuts many 
deep drainage channels so that the whole area is 
crossed by a network of tidal creeks. These can 
provide only low flood levels and carry fine mate 
rials. Therefore, in the tidal floodplain only nar 
row levels of very fine sand and silt are formed, 
and silt or very fine silt and clay are deposited in 
the extensive basins. Due to a dark appearance of 
vegetated levees and a light appearance of bare 
basins, a very dense pattern df levees and basins is 
clearly seen on band-5 images. This pattern differs 
in detail in different locations mainly due to types 
of material in relation with topographical differen 
ces. Analysing the landscape pattern on band-5 
images and the colours seen on both images of 
different dates, the tidal floodplain can be 
subdivided into a number of lithologic units. 
A very fine sand and silt: On the 1973 band-5 
image (fig. 2) a pattern produced by dark colours of 
vegetated densily developed small levees and light 
colours of bare, small and narrow basins, is visible 
in the areas in the North and Northwest of the tidal 
floodplain. The rest of the floodplain is more or 
less uniform dark in colour without a distinguish 
able pattern. Bare basins and vegetated levees 
produce a uniform light colour on the band-7 image, 
so that a pattern cannot appear either. The light 
appearance of basins on both visible and near- 
infrared images of different dates are typical 
indications of sand or silt. 
Clay, peat and high amounts of organic matter: In 
between the light colour of the tidal floodplain 
some dark areas are seen on the band-5 image of 
1974 (fig. 2). The dark colours are due to the dark 
appearance of both vegetated levees and bare 
basins. Although vegetated levees are white on 
near-infrared images of different dates, basins, 
however, appear partly dark-grey and partly as 
black patches on the band-7 images of 1974 and 
1975. These dark-grey and black colours indicate an 
absorption of high amounts of energy in both visi 
ble and near-infrared regions of the electromagne 
tic spectrum which is characteristic for highly 
saturated delta materials such as peat, sediments 
rich in organic matter and clay. These areas might 
be relatively lower parts of the tidal floodplain 
and therefore peat layers and abundant organic 
material accumulated in the past, partly buried by 
later sedimentation, can be seen now only as dark 
patches. The later sedimentation might contain high 
amounts of clay which being always wet in the lower 
areas does not show remarkable changes with time 
and appears with the same dark-grey colours on both 
visible and near-infrared images of different 
years. 
Silty-clayey deposits: After the identification 
of two small units in the tital floodplain the 
remaining area shows a nicely developed pattern on 
the band-5 image of 1974 and even better on that of 
1975. This pattern is produced by white colours of 
wide basins and dark colours of vegetated long 
levees along the large channels, and light-grey 
colours of relatively narrow basins and dark co 
lours of vegetated short levees far from the large 
channels. However, due to the gradual change of 
size and colours of basins and levees, a boundary 
between the two areas cannot be drawn. The basins 
along the large channels, white on visible images, 
appear also white on near-infrared images while 
dark-grey basins, far from the large channels, 
appear darker. It is therefore assumed that fine 
sediments accumulated in extensive basins of the 
tidal floodplain with silty basins along the large 
channels and clayey basins at a greater distance 
from the main channels. 
Mud bands: Besides the tidal floodplain some 
tidal creeks can also be seen along the Meghna 
River. They are partly covered by green vegetation 
and partly bare. The bare areas appear uniform dark 
grey on band-5 and blackish on band-7 images of 
1973 and 1974. The dark and very dark appearance of 
areas on images of different years typically indi 
cate highly saturated organic matter, peat and clay 
as explained previously. Considering their location 
in the riverbeds, these areas might consist of "mud 
bands". 
1.4 Subdivision of composite floodplain 
The composite floodplain, which is found between 
the tidal and river floodplains, is situated far 
from the main rivers and active channels. Except 
the extension of the Madhumati River and a few 
tidal creeks, only a number of man-made channels 
can be identified because of their geometry. The 
dark colours on band-7 images of different years 
indicate its continuously saturated condition. As 
mentioned previously, rainwater must have played an 
important role in the development of the composite 
floodplain, besides the effects of tidal and river 
floodings. One of the consequences of flooding by 
rainwater is that organic matter rather than min 
eral sediments develop in perennially wet basins 
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