The geographical location of Karimganj district is between longitudes 92°15' and 92°35' east and latitudes 24°15' and 25°55' North.
The district is bounded on the North by Bangladesh and Cachar district; on the South by Mizoram and Tripura states, on the West by Bangladesh and Tripura and on the East by Hailakandi district.
Located strategically, the district shares 92 Kms. of International Border with the neighbouring country of Bangladesh. 41 Kms of this is demarcated by the river Kushiara while 51 Kms is land border. On some stretches, there is no natural geographical demarcation for the border which cuts across open agricultural or grazing fields. However, on most parts, the international border with Bangladesh is marked by either the river Kushiara, or the sub-mountain tracts of the Adamail range. In a sense, Karimganj, along with the neighbouring district of Cachar demarcates the frontier between the plains of the Padma-Meghna basin and the hilly North-east India.
The Chhatachura range that starts from the south-east border, forms the whole length of border with Hailakandi district. The summit of the range is called the Chhatachura peak and its height is 2087 feet above the sea-level. The hills gradually decline in height and in the middle section, which bears the name Sarashpur, are only 1000 feet above the sea-level near the Barak river. At the lowest level, where they are known as the Badarpur hills, the average height is about 500 feet. The Chhatachura range is about 50 miles from north to south and at some parts, 13 miles in breadth.
The Adamail or Patharia range marks the western border of the district forming the international border with Bangladesh. Running from the south to the north, its length is about 28 miles and breadth about 7 to 8 miles. The highest point of the range is about 800 feet above sea-level.
The third hilly range crossing through the district is the Duhalia range, also called the Pratapgarh range. It runs through the mid-south of the district demarcating the Longai Valley and the Chargola valley. The length of the range in the district is about 28 miles with highest peak at 1500 feet above sea-level,
Besides these main ranges, the plains of the district are also dotted with hillocks and forests. The north and north-eastern portion of the district are mainly plains whereas the South and South-western parts are mainly covered with forest.
The river Barak enters the district through its north-eastern corner near Badarpurghat and after traversing a length of seven miles upto a place called Haritikar near Bhanga, is divided into two branches - namely, the Kushiara and the Surma. From the point of bifurcation, the Kushiara flows westwards to Bangladesh forming the northern boundary of the distict. The town of Karimganj is situated on the bank of this river. The old name of the Kushiara near Karimganj town was Bagali. In Bangladesh, the river is again divided near Bahadurpur in Moulavi Bazar district, the northern branch assuming the name Bibiyana and the southern branch, Shakha Barak. The Bibiyana is later merged with the Surma rver near Markuli steamer station in Habiganj district of Bangladesh, assuming the name Kalni, and then Bhera Mohana, and ultimately this huge combined stream merges with the great river Meghna of Bangladesh. The southern stream of Kushiara resumes the original name Barak or Shakha Barak and flows in a south-westerly direction through Habiganj district and finally falls into the old bed of the Brahmaputra near Bhairab Bazar in Maimansingh District (Bangladesh).
The Longai river originates in the Jampai Hills of Tripura state and travelling a course of northerly direction, turns south-west near Longai Railway station near Karimganj town. Near Latu village, it enters Bangladesh and then flows to meet the Hakaluki Haor (Haor = Atoll-like span of water) which absorbs the entire inflow during the winter. During the rainy season, an outlet springs out which ultimately merges with the Kushiara near Fechuganj in Sylhet district (Bangladesh).
The Singla river originates from Mizoram state and taking a northward direction, it falls in Sonbill Haor wherefrom the stream emerges bifurcated forming two rivulets - Kachua and Kakra.
The Kushiara and the Longai are perennial rivers, whereas the others dry up during the winter.