The Ganga divides Bihar into how many physical plains?

Two (North & South Bihar plains)

Most of Bihar's annual rainfall is received during which season?

Southwest monsoon (June–September)

Post-monsoon (October–November)

Geography of Bihar: Rivers & Physical Divisions

To understand Bihar, understand the Ganga. One river shapes the state's farming, its floods, its cities and its history. If you can picture the map — the Ganga in the middle, Himalayan rivers rushing down from the north, and rain-fed rivers rising from the southern plateau — you can answer almost any Bihar geography question.

Location & Physical Divisions

Bihar is a landlocked state in eastern India, lying almost entirely in the fertile Middle Ganga Plain. The Ganga flows across it from west to east, splitting Bihar into two natural halves: the North Bihar Plain and the South Bihar Plain.

Bihar and its neighbours — Nepal to the north (international border), and the Ganga running through the middle.

The two physical divisions
Feature	North Bihar Plain	South Bihar Plain
Rivers come from	The Himalayas (Nepal) — snow-fed, perennial	The southern plateau — rain-fed, seasonal
Main problem	Severe floods almost every year	Relatively drier; some drought
Key rivers	Ghaghara, Gandak, Bagmati, Kamla, Kosi, Mahananda	Sone (largest), Punpun, Falgu, Kiul

The River System

The Ganga divides Bihar; Himalayan rivers pour in from the north, peninsular rivers rise from the south.

💡 Think of it like this

Picture the Ganga as the spine of a fish. The bones on the top side (north) — Gandak, Bagmati, Kosi — are long and come down from the tall Himalayas, so they carry snow-melt all year and flood often. The bones on the bottom side (south) — Sone, Punpun, Falgu — are shorter and only fill up in the monsoon.

The Kosi is the most important — and most feared — river of north Bihar. Because it repeatedly changes its course and floods vast areas, it is called the "Sorrow of Bihar". It rises in Nepal/Tibet.
The Sone is the largest river of south Bihar; it joins the Ganga near Patna (at Maner) after flowing north.
The Gandak (also called Narayani) and the Ghaghara are other major north-bank tributaries of the Ganga.

⚠️ Exam trap

Do not confuse the "sorrows": the Kosi is the "Sorrow of Bihar", while the Damodar was the "Sorrow of Bengal". Also remember direction — north-bank rivers flow southward into the Ganga; south-bank rivers flow northward into it.

Boundaries, Extent & Bihar's Only Hills

Bihar is bordered by Nepal to the north (an international border), Uttar Pradesh to the west, Jharkhand to the south, and West Bengal to the east. It is a landlocked, almost entirely plain state — but it does have a few small hilly/forested tracts that BPSC likes to test precisely because they are exceptions to the "flat plain" rule.

In the extreme north-west, in West Champaran district, the Someshwar and Dun hill ranges (an extension of the Himalayan foothills, or Siwaliks) rise along the Nepal border — this forested tract is home to the Valmiki Tiger Reserve, Bihar's only tiger reserve.
In the south-west, the Kaimur Plateau (an extension of the Vindhyan range) covers parts of Kaimur and Rohtas districts — the only true plateau/upland tract in present-day Bihar.
Smaller rocky hillocks also appear around Rajgir and Gaya (linked to the ancient history of Magadha) — the low Rajgir hills encircling the old capital, and the Barabar hills near Gaya famous for the Barabar Caves.

🎯 Why it matters

These hill tracts matter for exams precisely because Bihar is otherwise almost entirely alluvial plain — "Which district has Bihar's only tiger reserve?" (West Champaran) and "Which range/plateau lies in Kaimur-Rohtas?" (Kaimur Plateau, a Vindhyan extension) are recurring BPSC-style questions.

Taming the Kosi: Bihar's Flood-Control Story

Because the Kosi's repeated course-changes have flooded huge areas of north Bihar for centuries, India and Nepal undertook a major joint flood-control effort: the Kosi Project, based on the Indo-Nepal Kosi Agreement (1954). Its centrepiece is the Kosi Barrage at Bhimnagar (in Nepal, just across the border), completed around 1963, along with hundreds of kilometres of embankments on both banks meant to confine the river to a fixed channel.

⚠️ Exam trap

A frequent exam trap: the embankments were meant to control floods, but by confining silt within the channel they have also raised the riverbed over time, which is why the Kosi still breaches its embankments occasionally (as in the major 2008 Kosi flood/breach at Kusaha in Nepal) — do not assume the problem is fully "solved".

Climate & Soil

Climate: subtropical monsoon — hot summers, a rainy monsoon (June–September, mostly from the Bay of Bengal branch), and mild winters. Most rainfall comes in the monsoon, which is why floods and farming both depend on it.
Soil: mostly rich alluvial soil deposited by the rivers — ideal for crops like paddy, wheat, maize, pulses and sugarcane. Newer alluvium (khadar) lies near the rivers; older alluvium (bhangar) lies away from them.

🎯 Why it matters

This geography explains Bihar's economy and its problems in one line: fertile alluvial soil makes it a strong agricultural state, but its dependence on flood-prone Himalayan rivers makes flood management a permanent governance challenge — a favourite theme for BPSC Mains.

Key takeaways

The Ganga flows west→east and divides Bihar into the North Bihar Plain and South Bihar Plain.
North-bank rivers (Himalayan, perennial, flood-prone): Ghaghara, Gandak, Bagmati, Kamla, Kosi, Mahananda.
South-bank rivers (peninsular, rain-fed): Sone (largest), Punpun, Falgu, Kiul.
Kosi = "Sorrow of Bihar" (changes course, floods north Bihar). Damodar = "Sorrow of Bengal".
Soil is mostly fertile alluvial (khadar = newer, bhangar = older); subtropical monsoon climate → strong agriculture but chronic flood challenge.
Bihar borders Nepal (north, international), UP (west), Jharkhand (south), West Bengal (east). Only hilly tracts: Someshwar/Dun Range (West Champaran — Valmiki Tiger Reserve) and Kaimur Plateau (Kaimur-Rohtas, a Vindhyan extension); small hillocks at Rajgir & Barabar (Gaya).
Kosi Project: based on the Indo-Nepal Kosi Agreement (1954); Kosi Barrage at Bhimnagar (Nepal) completed ~1963, with embankments on both banks. Embankments raised the riverbed over time — the river still breaches occasionally (2008 Kusaha breach).

Practice questions (10)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

The Ganga divides Bihar into how many physical plains?
1. Two (North & South Bihar plains)
2. Three
3. Four
4. It does not divide Bihar
Show answer
Answer: A — Two (North & South Bihar plains)
Flowing west to east, the Ganga divides Bihar into the North Bihar Plain and the South Bihar Plain.
Which is the largest river of the South Bihar plain?
1. Kosi
2. Gandak
3. Sone
4. Bagmati
Show answer
Answer: C — Sone
The Sone is the largest south-bank tributary of the Ganga in Bihar, joining it near Patna (Maner).
The river known as the "Sorrow of Bihar" is:
1. Sone
2. Kosi
3. Damodar
4. Punpun
Show answer
Answer: B — Kosi
The Kosi is called the "Sorrow of Bihar" because it frequently changes course and causes floods. (The Damodar was the "Sorrow of Bengal".)
Consider the following statements:
1. The rivers of North Bihar are perennial and originate in the Himalayas.
2. The rivers of South Bihar are mainly rain-fed and seasonal.
Which is/are correct?
1. Only 1
2. Only 2
3. Both 1 and 2
4. Neither 1 nor 2
Show answer
Answer: C — Both 1 and 2
Both are correct. North Bihar's Himalayan rivers are snow-fed and perennial; South Bihar's plateau rivers are rain-fed and seasonal.
The predominant soil type of the Bihar plains, ideal for agriculture, is:
1. Black soil
2. Red soil
3. Alluvial soil
4. Laterite soil
Show answer
Answer: C — Alluvial soil
The Bihar plains have rich alluvial soil deposited by the rivers — newer alluvium (khadar) near rivers, older alluvium (bhangar) away from them.
The Gandak river is also known as:
1. Narayani
2. Kamla
3. Mahananda
4. Karmanasa
Show answer
Answer: A — Narayani
The Gandak is also called the Narayani. It is a major north-bank Himalayan tributary of the Ganga.
Most of Bihar's annual rainfall is received during which season?
1. Winter (December–February)
2. Summer (March–May)
3. Southwest monsoon (June–September)
4. Post-monsoon (October–November)
Show answer
Answer: C — Southwest monsoon (June–September)
Bihar has a subtropical monsoon climate; most rain falls during the southwest monsoon (June–September), mainly from the Bay of Bengal branch.
Bihar's only tiger reserve, the Valmiki Tiger Reserve, is located in which district, along the Someshwar/Dun hill range?
1. East Champaran
2. West Champaran
3. Kaimur
4. Rohtas
Show answer
Answer: B — West Champaran
The Valmiki Tiger Reserve lies in West Champaran district, along the Someshwar and Dun hill ranges (an extension of the Himalayan foothills/Siwaliks) on the Nepal border.
The Kaimur Plateau, found in the Kaimur and Rohtas districts of Bihar, is an extension of which range?
1. Aravalli Range
2. Vindhyan Range
3. Satpura Range
4. Himalayan foothills (Siwaliks)
Show answer
Answer: B — Vindhyan Range
The Kaimur Plateau is an extension of the Vindhyan Range, and is the only true plateau/upland tract in present-day Bihar — distinct from the Himalayan-foothill hills of West Champaran.
The Kosi Barrage, a key structure in India-Nepal flood-control efforts on the Kosi, is located at:
1. Bhimnagar (Nepal)
2. Birpur (Bihar)
3. Kusaha (Nepal)
4. Purnia (Bihar)
Show answer
Answer: A — Bhimnagar (Nepal)
The Kosi Barrage is located at Bhimnagar in Nepal, just across the border from Bihar, and was completed around 1963 under the Indo-Nepal Kosi Agreement (1954).