This valley is 80 to 129 km. broad and about 725 km. long. The Brahmaputra which drains this valley emerges from the Abor Hills at Pasighat under the name of the Dihang and flows past Sadiya into the Assam Valley. Two big rivers, the Dibang from the north and the Luhit from the east meet the Dihang on its left bank near Sadiya.
Below Sadiya it is called the Brahmaputra. It flows majestically to the west till near Dhubri it turns to the south and after flowing for some kilometres southward, it enters Bangladesh. The Brahmaputra has deposited a few hundred metres thick alluvium over the underlying compressed sedimentary rocks in this region.
This region is known for frequent floods due to heavy rain over 250 cm. Devastating floods sometimes occur during the summer monsoon rainy season. The flood water may spread over half of the area of the valley causing damage to the standing crops of rice and jute and filling low-level areas.
The middle part of the valley stretching for about 145 km. west of Tezpur, is relatively dry and has less than 178 cm. of rainfall in year. Gauhati located in this relatively dry belt receives 163.7 cm. of rainfall a year. The summer monsoon rainy season is quite long in this region starting from June and ending in month of October. Vegetation varies from tropical evergreen forest in the upper Assam Valley to tropical rain evergreen forest in the middle and the lower Assam valley.
The percentage of the area sown more than once is fairly high and is 25% of the net area sown. There is the narrow strip of land which lies beside either bank of the Brahmaputra and its tributaries is called the Chapari. The ‘Chapari’ is infested with tall grasses and grows summer (ahu) rice which may be damaged considerably when rivers overflow their banks too early in this region.
This is a main crop of the Assam Valley. As much as three-fourths of the total cropped area in the valley is under rice. The winter rice crop which is sown in June and July and harvested in November and December, is the most important crop. Rice is mainly a rain-fed crop. The yield of rice per hectare is among the highest in India.
Tea is the most important crop in the Upper Assam Valley. Lakhimpur, Sibsagar and Darrang are main tea producing centre. These districts have 74 percent of the hectare under tea in Assam. Though area under tea is only 6 percent of the total cropped area, the region produces almost 90 percent of tea of Assam. Assam State is an important producer of tea and accounts for about 55 percent of the total tea produced in the county.
Rainfall is heavy and copious during the rainy season and is more than 178 cm. a year. Thus, the Upper Assam Valley has excellent climatic conditions for tea culture. Deep, rich, well-drained soil, fairly rich in phosphates and potash and organic matter is another factor for tea cultivation.
Jute is a commercial crop of this valley. It is raised mainly in the lower half of the valley. About 5 per cent of the total cropped area is under this crop in the Assam Valley. Its cultivation continues to the west where this jute belt merges itself into the jute growing area of Koch Bihar (West Bengal). Jute raised in fresh silt has soft fibre. Sown in March and April jute is harvested in July and August.
Lakhimpur and Sibsagar in upper Assam have petroleum. A refinery is located at Digboi. New oil-fields have been discovered at Naharkatia, Moran, Hugrijan, Rudrasagar and Lakwa. Oil from these fields is conveyed by pipelines to the refineries located at Nunmaati (near Gauhati), Bongaigaon (near Jogighopa) and Barauni (Bihar).
In Assam natural gas is being used as a fuel for industries and also as a raw material for the manufacture of fertilizers. It is now used by the tea processing industry and as a domestic fuel. A fertilizer factory based on gas is located at Namrup (near Naharkatia). There is petro-chemical complex based on gas at Namrup.
There are less number of large scale industries which are limited to the refining of oil at Digboi, Nunmati and Bongaigaon, and to the manufacture of fertilizers at Namrup and to the processing and packing of tea. Tinsukia, Mariani, Saikho-Waghet and Makum are important for sawing of woods and plywood.
The region is one of the important silk producing areas of India and it produces mainly non- mulberry silk of the varieties of muga and eri. A fine and glossy variety of silk known as pat is also produced. It is mulberry silk. This region produces 32 percent of silk of India.
The Assam Valley occupies only 22 percent of the total area of the Assam Region; it accommodates 61 percent of the total population of the region. In the Upper Assam Valley, the villages lying near the rivers are subjected to floods almost every year. The houses are, therefore, built on bamboo platforms supported by wooden posts.
Guwahati, Dibrugarh, Dhubri, Darranga and Nowgong are important town of this region.