Introduction
Bihar’s geography is dominated by the Gangetic Plain, a vast alluvial landscape shaped by the Ganga and its tributaries. This subtopic — “Bihar geography — Gangetic plains, rivers, floods” — is a core component of the BPSC Geography syllabus and has appeared consistently across examinations. The provided 19 previous year questions (from 2018 to 2025) reveal that BPSC tests both factual recall (area, height, drainage system) and applied understanding (flood causes, river characteristics, physiographic regions). The examination pattern includes direct one-liners, matching‑based questions, and location‑based reasoning.
The Gangetic plain of Bihar is not a uniform expanse. It comprises the North Bihar Plain (heavily flood‑prone, younger alluvium) and the South Bihar Plain (older alluvium, arable land). The rivers — Ganga, Son, Gandak, Kosi, Ghaghra, Bagmati, Kamla, Karmanasha — form a complex drainage network that dictates agriculture, settlement, and disaster vulnerability. Floods are a recurring phenomenon, particularly in the Kosi and Gandak belts, and understanding their geomorphological causes is essential.
This chapter will equip you with a deep, integrated understanding of the Gangetic plains, river systems, flood dynamics, and the associated human‑geography implications that BPSC demands. We will build from first principles, define every key term, examine PYQs in detail, identify patterns, and anticipate future question angles. By the end, you will be able to answer any question within this subtopic — factual, analytical, or matching — with confidence.
Core Concepts & Foundations
Before diving into Bihar‑specific details, we must establish a solid conceptual foundation. Each key term below is defined in 2–3 sentences and will be used throughout the chapter.
Gangetic Plain: A vast, flat tract of land formed by the deposition of sediment (alluvium) by the Ganga and its tributaries over millions of years. It is one of the most fertile regions in the world, supporting intensive agriculture. In Bihar, it covers nearly the entire state, divided into North and South portions by the Ganga River.
Alluvial Soil: Soil deposited by rivers, characterised by fine silt, clay, and sand. It is rich in potash but poor in nitrogen and phosphorus. In Bihar, over 90% of the land is covered by alluvial soil, which is classified into khadar (newer, flood‑plain alluvium) and bhangar (older, elevated alluvium).
Drainage Basin: The area of land from which all precipitation flows into a single river or river system. Bihar’s drainage is primarily part of the Ganga Basin, which includes the Ganga itself and its major tributaries: Ghaghra, Gandak, Kosi (left bank) and Son, Punpun, Kiul (right bank).
River Regime: The seasonal variation in a river’s flow, determined by the distribution of rainfall and meltwater. In Bihar, most rivers have a monsoonal regime — high discharge from June to September and low flow in winter and pre‑monsoon. This regime is the primary cause of annual floods.
Flood Plain: The flat, low‑lying area adjacent to a river that is inundated during high‑flow periods. It is built by repeated deposition of sediment. In Bihar, the flood plains of the Kosi, Gandak, and Bagmati are the most extensive and experience the worst flooding.
Oxbow Lake: A crescent‑shaped lake formed when a river meander is cut off from the main channel, leaving a water body. Kanwar Lake in Bihar is the largest freshwater oxbow lake in Asia, located in Begusarai district.
Diara Land: Low‑lying, flood‑prone land along the banks of the Ganga, particularly in the districts of Patna, Saran, and Buxar. These lands are extremely fertile but vulnerable to erosion and inundation.
Khadar and Bhangar: Khadar is the new alluvium deposited annually by floods, found in river flood plains. Bhangar is the older alluvium, above the flood level, often containing calcareous concretions (kankar). In Bihar, the North Bihar Plain is predominantly khadar, while the South Bihar Plain is a mix of bhangar and khadar.
Physiographic Region: A geographical area with distinct landforms, relief, and geological history. Bihar is divided into three major physiographic regions: the North Bihar Plain, the South Bihar Plain, and the Southern Hilly Region (including the Kaimur, Rohtas, and Rajgir hills).