The north of India has the largest rivers, the ones that flow out from the mighty Himalaya, and these usually get most of the attention from anglers. I will therefore buck the trend and start down south.
Down South: The Kauveri River
At quick glance at an Atlas is unlikely to give you much indication of the great fishing prospects to be had in southern India. It seems like there are a few artificial lakes, a few rolling hills and lots of cities. Not the usual territory for big and exciting fish. The Deccan Plateau however, holds a world-class surprise in the Kauveri River (Previously more familiar as the Cauvery River).
Stretching across the State of Karnataka and finally exiting to the sea on the Coromandal Coast the Kauveri River is home to the Humpback Mahseer (Tor Mussulah). These can reach 150 centimeters in length and an amazing 90 kilograms. They are renowned for the fighting power and even smaller specimens can bust tackle designed for deep-sea anglers. Another species, the Deccan Mahseer (Tor Khudree), is much smaller but a full-grown adult of 50 centimeters and perhaps 3 kilograms can still put up a good fight.
Several sections of the river are noted for their beats but the section South-west of Bangaluru (previously Bangalore) and East of Mysuru (previously Mysore) would be as good as any to try for a first visit. Fishing is best in the mornings and evenings, either from the shore or from a local coracle. There are established fishing camps and fishing guides to make a fishing holiday that more convenient.
Up North: The Ramganga and Kosi Rivers
Corbett National Park is situated to the North and East of New Delhi. This wilderness is more famous for its wildlife, especially its tigers, but deserves to be noted for one of Jim Corbett's other passions, fishing.
The Ramganga River is the largest permanent water source in the Jim Corbett National Park and hosts a wide range of wildlife. Anglers will be most interested in the Mahseer and Goonch (a type of catfish ... more about them later.)
The Kosi River forms a section of the Eastern border of the park. It holds its fair share of decent size Mahseer and is well worth checking out if in the vicinity.
The park can be reached quite easily by private car and has lots of comfortable accommodation. Fishing trips are best arranged through an agency as it is necessary to have the correct permits.
To the North-east: The Brahmaputra River
The mighty Brahmaputra should need little introduction. As one of the world's largest rivers, its name and general location should be familiar to most.
The Brahmaputra can be fished along much of its length in India. Mahseer are available in the upper reaches and fast-flowing tributaries. Giant catfish are known to exist but these are only likely to be taken by those with an abundance of patience and skill.
Choosing a section to fish along such a long river is difficult. The section between Gauhati (alternatively Guwahati) and Jorhat is relatively convenient and produces reliable catches. Further north, the section beyond Pasighat is more remote and would suit the truly adventurous angler looking for excitement and a challenge.
To the North-west: The Beas River
The Kullu Valley is a major tourist destination in Himachal Pradesh. The Beas River and its tributaries (particularly the Tirthan River and the Baspa River in the Sangla Valley) contain a variety of trout (more properly, snow trout) that give the game angler an opportunity to practice their sport in gorgeous mountain terrain.
Kullu has an airport which saves a long and sometimes frightening road experience. Travel around is best arranged in private vehicles for convenience. There are good guesthouse options in the area, some specifically catering for anglers. You may need to book well in advance for the more popular ones.
One more, for the Catfish fanatics: The Kali River
The Kali River has become infamous as the home of giant species of catfish that has allegedly taken to eating human flesh. These Goonch grow large enough to believe that they may occasionally take a piece of corpse cast into this holy river after a traditional cremation - but I am still not convinced that they are big enough or even inclined to take live human bait as the one story has it. Whatever the truth of that matter, the Kali River is an exciting venue for the catfish angler after an exotic fishing holiday abroad.