The Kosi barrage, built under the India-Nepal Kosi agreement of 1954, is situated at:

The correct answer is B, the Kosi. The Kosi earns the name because it brings an exceptionally heavy load of silt down from Nepal, drops it the moment it reaches the flat North Bihar plain, raises its own bed above the surrounding land and then breaks out into a new channel. Over about two hundred years the river swung roughly 112 kilometres to the west, so the flood did not return to the same tract but ruined a fresh one each time.

Option A, the Gandak, floods Champaran and Muzaffarpur badly but is controlled from the Valmikinagar barrage and has no such record of shifting. Option C, the Son, is a southern plateau river, seasonal and comparatively well behaved, and is remembered as the largest southern tributary of the Ganga rather than for floods. Option D, the Punpun, is a small Chotanagpur stream that joins the Ganga near Fatuha. For the paper, remember the pairing of sorrows: the Kosi is the Sorrow of Bihar, the Damodar the Sorrow of Bengal and the Hwang Ho the Sorrow of China.

The correct answer is A, Chausa in Buxar district. The Ganga comes into Bihar from Uttar Pradesh near Chausa, flows from west to east for about 445 kilometres and leaves the state for West Bengal near Rajmahal, splitting Bihar into the North Bihar plain and the South Bihar plain on the way.

Option B, Maner, is the place west of Patna where the Son, the largest southern tributary, joins the Ganga, so it is a confluence inside the state and not the point of entry. Option C, Kursela in Katihar district, is where the Kosi meets the Ganga in the east of the state, again a confluence and not an entry point. Option D, Rajmahal, is the exit: the Ganga passes into West Bengal there, and papers often swap the entry and the exit to see whether a candidate has learnt both. Chausa is also worth remembering for history, as the site of the battle of 1539 in which Sher Shah Suri defeated Humayun.

The correct answer is B, Sonpur, opposite Patna. The Gandak rises in Nepal, where it is called the Narayani or the Sapt Gandaki, enters Bihar at Valmikinagar in West Champaran and flows south-east to meet the Ganga at Sonpur, across the river from Patna. The confluence is the site of the Sonepur cattle fair, which makes the pair easy to remember.

Option A, Doriganj in Saran district, is the confluence of the Ghaghara, the river that comes from Nepal as the Karnali and forms part of the western boundary of Bihar. Option C, Khagaria, is where the Burhi Gandak reaches the Ganga, and the similar name is exactly the trap the question is built on: the Gandak and the Burhi Gandak are different rivers with different mouths. Option D, Kursela in Katihar district, is the confluence of the Kosi. Learn the four mouths as a set, because match-the-following questions use all of them together.

The correct answer is B, the Son. The Son rises at Amarkantak in Madhya Pradesh, the same plateau that gives birth to the Narmada, flows north-east through Rohtas and joins the Ganga near Maner, a little west of Patna. It is the largest of the rivers that reach the Ganga from the south, and the Indrapuri barrage at Dehri-on-Sone diverts its water into the Sone canal system, one of the oldest canal networks in India.

Option A, the Punpun, is a much smaller Chotanagpur stream joining the Ganga near Fatuha, south-east of Patna. Option C, the Kiul, drains the Jamui and Lakhisarai tract and meets the Ganga near Surajgarha. Option D, the Chandan, comes from the Jharkhand hills into Banka and Bhagalpur. All three are southern rivers, so the question is really asking which is the largest, and the Son is also the second longest southern tributary of the Ganga in the country after the Yamuna.

The correct answer is A, Begusarai. Kabartal, also called Kanwar Jheel, lies in Begusarai district and is an ox-bow lake left behind by the Burhi Gandak. It was listed as a wetland of international importance under the Ramsar Convention in 2020, the first site in Bihar to be listed, and it is often described as the largest freshwater ox-bow lake in Asia. It is an important halt for migratory waterbirds on the Central Asian Flyway.

Option B, Jamui, is the district of the Nagi and Nakti bird sanctuaries, two irrigation reservoirs that were later added to the Ramsar list, and it is the distractor most often used against Begusarai. Option C, Katihar, has the Gogabil wetland, protected as a community reserve, and is where the Kosi meets the Ganga. Option D, Samastipur, has chaur wetlands of its own but no Ramsar site. The pairing to memorise is Kabartal with Begusarai and 2020.