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IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002 
flooding usually occurs when local monsoon rainfall is 
especially heavy and prolonged which results in more serious 
flooding. 
3. DATA AND METHODOLOGY 
Survey of India topographical maps on 1:50,000 scale covering 
the study area in part of sheet No. 83I/10,11,12,14,15 & 16 
surveyed in (i) 1916-17 & 1918-19 and (ii) 1963-64, 1968-69 
and 1969-70 were used for preparing Brahmaputra river 
configuration map, base map and for carrying out ground truth 
work in the study area. 
IRS-1A/IB, LISS-II Black & White Band 4 geocoded data on 
1:50,000 scale of 20.11.1988; 12.01.1990; 08.12.1990; 
10.06.1992; 25.12.1992; 1/23.12.1993; 14.02.1995; 01.02.1996; 
31.10.1997; and IRS-1C/ID, LISS-III false colour composite 
geocoded data on 1:50,000 scale of 24.02.1997; 06.08.1998 and 
02.11.1998 have been used for visual interpretation and 
mapping of different fluvial features such as braid bar with 
sand, braid bar with grasses, submerged braid bar, bank bar 
with sand/bank bar with grasses, flood plain, braiding pattern of 
Brahmaputra river and other associated features such as 
perennial river/stream, dry river/stream, old meander, oxbow 
lake, pond, lake, swampy/marshy land and surface waterlogged 
land on the basis of image characteristics including tone, 
texture, shape, size, pattern and its association. Brahmaputra 
river configuration maps of 1988 to 1998 period on 1:50,000 
scale have been prepared to understand the dynamic behaviour 
of the braiding channels with braid bars of the Brahmaputra 
river and compared with the Brahmaputra river configuration 
maps of 1916-19 and 1963-70. 
The study area was visited in the month of December, 1997 and 
a traverse was taken through boat along braided channel in the 
Brahmaputra river stretch in the vicinity of Bogibilgaon 
alignment site for collection of ground truth data and checking 
of interpreted maps. Collateral data collected from different 
Organization have also been utilized during the course of the 
study. Despite gigantic efforts and colossal expenditure in 
building embankments on both sides of the Brahmaputra river 
stretch and soil conservation measures taken in the area, the 
Brahmaputra river continues to wreck havoc by uncontrollable 
floods year after year. Past records indicated that catastrophic 
floods in Brahmaputra river valley occurred in 1950, 1954, 
1962, 1966, 1972, 1973, 1977, 1978, 1983, 1984, 1987, 1988, 
1991, 1993, 1995, 1996 and 1998. The Brahmaputra river near 
Dibrugarh attained highest water level at 106.20 m on 
10.07.1991, 105.25 m on 25.07.1992, 105.84 m on 06.07.93, 
104.86 m on 25.06.1994, 105.46 m on 16.06.1995, 105.73 m on 
19/07/1996, 106.02 m on 09.07.1997 whereas lowest water 
level recorded at 102.05 m on 31.01.1991, 102.43 m on 
14.01.1992, 102.12 m on 11.02.1993, 102.01 m on 24.02.1994, 
102.05 m on 16.02.1995, 102.03 m on 22.02.1996. 
4. RESULTS AND DISCUSSIONS 
The upper Assam valley is an intermontane platform basin of 
tertiary sedimentation. It is surrounded by orogenic belt of 
eastern Himalaya in the north and by Naga-Lushai orogenic 
belt to the south-east. The present structure and tectonic set up 
is a resultant of basement, Naga hills and Himalayan tectonics. 
Total thickness of the alluvium is reported to be about 7000 m 
(Chandsarkar, 1981). The Brahmaputra river lies in one of the 
most active tectonic zones of the world and experiencing the 
effects of earthquakes. The great earthquake (M 8.7) of August 
487 
15, 1950 in the region caused a landslides in the hills, stopping 
the flow temporarily and rapid accumulation of enormous 
volume of sediments in the channel by increasing the intensity 
of flood, bringing several changes in the river channel and a 
rise of river bed at Dibrugarh by about 2.5 to 3 m. The main 
cause of increased sediment load in the river is also attributed 
to the accelerated rate of soil erosion in the catchment area, 
shifting cultivation and stream bank cutting etc. Overloading of 
a stream flow with sediment normally results in higher 
width/depth ratio which is known to be the most important 
cause for the braided planform. In a channel, if the sediment 
load is higher than its transport capacity, widening and 
shallowing of the cross section takes place. The wide and 
shallow channel is unable to maintain uniform discharge 
distribution over its cross section. The flood discharge and 
heavy sediment influx brought, in influences the formation of 
braiding channels. The planform of braided rivers can change 
radically with flow stage. The annual hydrograph of the 
Brahmaputra river is characterised by low flows during the 
winter season and high flows during the summer season. 
A 45 km long stretch of the Brahmaputra river between 
Dibrugarh and Dighaligaon was studied for understanding the 
behaviour of Brahmaputra river configuration from 1916-19 
and 1963-70 period topographical maps and that of multidate 
satellite data of 1988 and 1998 period. Analysis of satellite data 
of different period provided a synoptic view of the braiding 
channel pattern and its braid bars. The typical braided nature of 
the Brahmaputra river is attributed to the fact that the high bed 
load is deposited within the river channel and on the banks in 
the form of sand mounds in response to the low carrying 
capacity of the channel. Most of the braid bars get submerged 
during high flood period. Some of them are even vanished 
whereas many new braid bars have come up due to continuous 
deposition and as scour due to excessive bank erosion and 
migration of active channels on banks. When the river is in 
spate, it carries away as much as 30 m of land in a single day 
and almost 1.5 km. of width of bank can disappear in erosion in 
monsoon season. It has been observed that the maximum 
erosion is experienced during the falling stages on any 
particular flood. In last eight decades, the braiding channels 
with braid bars of the Brahmaputra river have shown drastic 
changes, the main channel is divided into several active 
channels which are interlaced, converging and diverging in 
nature. The shape, size and positions of braid bars and width of 
the each braiding channels have changed from time to time as 
evidenced from analysis of multi date satellite data of 1988 and 
1998 period. Dissection of these braid bars during wanning 
flow produces the braided channel pattern. The river exhibits a 
widely varying discharge, easily eroded banks and highly 
mobile bed materials consisting of medium to fine grained 
sand. The river shifts frequently within its braid belt, adopting 
and abandoning numerous anabranches at all levels within its 
braided system. The Brahmaputra valley around Dibrugarh is 
almost a flat level plain with very little slope from NE to SW 
direction with a fall of about 10 cm per km. The width of 
Brahmaputra river belt varies from 4.95 km (1916-19) to 8.25 
km (1963-70 & 1988) near Dignalagaon, 7.2 km (1916-19) to 
8.75 km (1998) near Bogibilgaon and 7.9 km (1916-19) to 6.9 
km (1990) near Kathaiguri village. 
Almost all braid bars and braiding channels are unstable in 
nature. As per Brahmaputra river configuration map of 1916- 
19 period, the total area of the Brahmaputra river bed under 
study is measured as 25909.04 ha (+ data gap area of 1909.63 
ha), out of which 52 patches of braid bar with sand occupied an 
area of 8660.21 ha, 44 patches of braid bar with grasses