Keywords: Planform, Remote Sensing, Erosion, Sinuosity, Migration Rate

Abstract
Cartographic evidence (Rennell, 1776) shows that the Gorai River has been a major distributary of the Ganges river system since at least 200 years ago. It was a perennial river until the end of the last decade. The Gorai River carries almost 8 to 12% of the discharge of the Ganges River. This river is the main source of fresh water inflow into the southwest region of Bangladesh. The importance of this river varies with time and is governed mainly by the planform of its off-take with the Ganges River. The Farakka Barrage inside India, 18 km upstream of the international border, became operational in 1975. Since then the barrage has been diverting a substantial amount of dry season flow of the Ganges River through the Hoogly River and has thus initiated an irreversible process of deterioration of the Gorai River flow.

In response to the changes in the hydraulic regime, morphological characteristics of the river have been changing as well. Changes in planform characteristics have been addressed in this study using three sets of Landsat images (MSS-1973, 1984 and TM-1987) supported by hydrologic data. The images were classified with unsupervised classification and knowledge based thresholding to produce land, sand and water classes and the time series data was analyzed to characterize erosion, accretion and planform changes. The changes of sinuosity, rates of bank erosion and meander migration were derived from the image analysis. These were found to relate to the changes in the hydraulic regime. Morphologically, the river can be characterized into five distinct reaches. Response of each reach is different in magnitude and is mainly governed by physiographical characteristics and distance of the reach from the off-take. The images of low resolution (80m x 80m to 30m x 30m) only can be used to identify the morphological changes of dynamic reaches of the river, rate of changes of different parameters of which are in the scale of tenths to hundredths of meters. Using the images, it was also possible to derive tools for predicting the meander bend cutoff, bank erosion and accretion rate and changes of sinuosity.

Introduction
As it has done for most of the Holocene, the Bengal Delta is continuing to evolve. Much evidence is available of the changes that have taken place in the delta over the past few thousand years with the changes of base level (Umitsu, 1990; Goodbred, 1998). Rennel (1776) map give evidence that the main flow of the Ganges was following the present course of the Arial Khan River and the Gorai River was one of the main distributaries of the Ganges River. After the avulsion of the Jamuna River from its course along the eastern side of the Pleistocene Madhupur tracts to its present course, the combined flow of the Jamuna and Ganges moved further in the southeast direction and followed the present course as Lower Meghna (Figure 1).

Figure 1 Map of Bangladesh showing the study area
A recent study (EGIS, 1999) shows that during the last two and a half decades the annual flow volume of the Gorai River is declining. By using remote sensing technology an attempt has been made to identify the characteristics of the morphological changes of the river in response to this reduced flow volume. The understanding on the response of the river can be used to predict the changes of the river morphology in the coming decades. This study was carried out within the framework of Environment and GIS Support Project for Water Sector Planning (EGIS II) a project of the Ministry of Water Resources of Bangladesh.

The Ganges River
The average flood discharge of the river is approximately 50,000 m³/s. The discharge is mainly contributed by the snowmelt of the Himalayas and monsoon rainfall. In general, the flood peak occurs between the end of August and early September. The bed material of the river consists of fine sand. The river is very dynamic and the channel of the river shifts between meandering and braided.

Before mid 1970s the river flowed without any major intervention along its course. Through the implementation of the Farakka Barrage in 1975, India started to divert a part of dry season flow of the Ganges River through the Hoogly River. During the post-Farakka period major changes have occurred in the dry season flow, especially between January and May. During the pre-Farakka period the minimum monthly average flow was 1,500 m³ /s. The recorded minimum monthly average flow was 170 m³ /s in April, 1995 (EGIS, 1999). However, the post-Farakka flood flows found to be of the same as the pre-Farakka flood flows.

Delft Hydraulics/DHI (1996) identified an important change in the duration of the flood recession between the pre- and post-Farakka periods. They observed that the period of the flood recession, which is very much relevant to the dry season flow of the Gorai River, has been reduced to half of its post-Farakka period. Delft Hydraulics/DHI and Halcrow (1993) both suggested that quicker recession might not leave sufficient time for the river to scour out a natural low water channel that used to maintain the perennial flow into the Gorai.

Figure 2 Different reaches of the Gorai River