The Tista also originates in the Himalayas and enters Bangladesh from the north. During the rainy season the water from the system flows back into the Meghna depression, creating a vast sheet of water. All are swift-flowing, relatively small rivers of the hill country that frequently cause flash floods. The harbor of the Chittagong is located at the mouth of the Karnaphuli. Climate Bangladesh has a tropical monsoon type of climate, with heavy summer rain and high summer temperatures.
Winters are dry and cool.
Rain begins to fall in April, the hottest month. Farmers then begin to plow their fields for the main crop planting. These heavy rains prevail for two to three months and continue more moderately into November. Destructive floods every two or three years are common in Bangladesh. Between 1960 and 1970, nine cyclones accompanied with floods hit Bangladesh during the October through December period.