Thanjavur district is essentially a deltaic plain comprising of old and new delta. The old delta has a net work of canals and channels of the river Cauvery and Vennar. Upper portion of this new delta area is irrigated by Grant Anaicut canal. Tapping of ground water is done considerably in this area to advance the first cropping season Kuruvai to avoid damage due to North East monsoon and to accommodate the two crops namely Kuruvai and Thaladi.
The soils of new deltaic area are amenable to wide variety of crops such as Coconut, Mango, Guaua, Pulses, Cotton, Gingelly, Groundnut, Banana etc. Cultivation of oilpalm and Soyabean is also carried out in this district wherever assured water supply and drainage facilities are available.
The major crops cultivated in Thanjavur district are Paddy, Pulses, Gingelly, Groundnut and Sugarcane. The minor crops like Maize, Soyabeans, Redgram are also grown. Paddy is the principal crop grown in three seasons viz. Kuruvai, Samba and Thaladi. Pulses like Blackgram, Greengram and cash crops like Cotton and Gingelly are grown in rice fallows.
In new delta area, the Groundnut is the principal crop . Sugarcane is cultivated both in new delta and old delta. Banana is primarily grown in Padugai lands.
SOIL: The geological formation of Thanjavur district is made up of cretaceous, Tertiary and Alluvial deposits and the major area is occupied by the Alluvial and Tertiary deposits. The cretaceous formation occur as a small patch in West and South-West of Vallam. These formations have a very thick lateritic cap consisting of impure lime stones and sand stones of silt, clay calcarious and argillaceous variety, in the coast, these formations are over lain by Cuddalore sand stone of tertiary age. The Cuddalore sand stone of Tertiary age are well developed as best seen, West of Grant Anaicut canal and near Orathanadu. These sand stones are covered by a thin layer of wind brown sandy clays, unconsolidated sand, clay bound sands and mottled clays with the lignite seams. This tertiary formation is invariably capped by laterite. In the east, the alluvial deposits of the river Cauvery and its tributaries lie over the Tertiary sand stone. They consist of sands, gravelly sands, clays and sandy clays. The thickness of these formations ranges from 30 Mt. to 400 Mt.
RIVERS: The river Cauvery and its tributaries are the most remarkable feature of Thanjavur District. The three minor tributaries , Palar, Chennar and Thoppar enter into the Cauvery on her course, above Mettur , where the famous dam has been constructed. The Mettur dam joins the Sita and Pala mountains beyond that valley through which the Cauvery flow, upto the Grand Anicut. The dam in Mettur, impounds water not only for the improvement of irrigation but also to ensure the regular and sufficient of water to the important Hydro-Electric generating station at Mettur. The river further runs through Erode district where river Bhavani merges with it. While passing through Erode, two more tributatries vice versa. Noyyal and Amaravathi join it before it reaches Thiruchirappalli district. Here the river becomes wide, with a sandy bed and flows in an earterly direction till splits into two at upper anicut about 14 kilometres west of Thiruchirappalli. The northern branch of river is called the Coleroon while the southern branch retains the same name Cauvery and then goes directly eastwards into Thanjavur District. These two rivers join again and form the Srirangam island near Thiruchirappalli.
The Chola king, “Karikalan” has been immortalised as he has constructed the bank for the Cauvery all the way from Puhar (Kaveripoompattinam) to Srirangam. It was built as far back as 1,600 years ago or even more. On both sides of the river are found walls spreading to a distance of 1,080 feet. The dam Kallanai on the border between Tiruchirappalli and Thanjavur constructed by him is a superb work of engineering, which was constructed with earth and stone and has stood the vagaries of nature for hundreds of years. In 19th century, it was renovated in a bigger scale. The name of the historical dam has since been changed to “Grand Anicut” and stands as the head of great irrigation system in the Thanjavur district. From this point, the coloroon runs north-east and discharges herself into the sea at Devakottai, a little south of Parangipettai. From river Coleroon, Manniar and Uppanai Branch of at lower Anicut and irrigates a portion of Mayiladuthurai taluk and Sirkazhi taluk. After Grand Anicut, the Cauvery divides into numerous branches and cover the whole of the delta with a vast network of irrigation channels and gets lost in the wide expanse of paddy fields. The mighty Cauvery river here is reduced to an insignificant channel and falls into the Bay of Bengal at the historical place of Poompuhar (Kaveripoompatinam) about 13 Kms north of Tharangampadi. The river Cauvery flows the entire district in different names through its tributaries and branches viz., Grand Anicut canal, Vennar, Pannaiyar, Koraiyar, Vettar, Kodamuritiyar, Thirumalairajanar, Arasalar, Veerasozhanar, Mudikondan, Noolar, Vanjiar, Vikaraman, Nattar, Kirtimanar, Nandalar, Majalar, Mahimalayar, Palavar, Cholasudamani, Puthar, Valappar, Vadavar, pamaniar, Mulliyar, Ayyanar, Adappar, Harichandranathi, Vellaiyar, Pandavaiyar, Odambogiyar, Kattar, Kaduvaiyar and all these branch off into a number of small streams.