In which of the following rivers, the upper course contains fresh water but saline water flowing at the lower part ?

Question

UnlockIAS · Accepted Answer

Correct — B, the Luni river. The Luni rises in the Aravalli hills near Ajmer — it is called the Sagarmati in its first stretch — and runs about 495 km south-west across Rajasthan before dissipating in the marshes of the Rann of Kachchh. In its upper course, through Ajmer, Nagaur and Pali, the water is sweet and is used for irrigation. Below that, roughly from Balotra in Barmer district onwards, it turns brackish and finally frankly saline. Three things do it: the river crosses salt-encrusted desert soils and saline groundwater, evaporation in the arid Thar concentrates whatever dissolved salt it carries, and it has no outlet to the sea to flush itself. The name records the fact — Luni derives from the Sanskrit lavanavari, salt water. It is the standard textbook example of a river that is fresh above and salt below.

Why the other options are wrong — (a) The Barak rises in the Manipur hills and flows through one of the wettest parts of India — Manipur, Mizoram and the Cachar plain of Assam — before entering Bangladesh, where it splits into the Surma and the Kushiyara and eventually forms the Meghna. Abundant rainfall keeps it fresh throughout its Indian course; there is no arid tract, no salt-rich soil and no evaporative concentration of the kind that makes the Luni saline.  (c) The Ghaggar is Rajasthan's other famous inland river — a seasonal, monsoon-fed stream often identified with the Vedic Saraswati, which simply loses itself in the sands of the Thar near Hanumangarh and Anupgarh. It is remembered for drying up and disappearing, not for turning salty; the salinity description belongs to the Luni.  (d) There is a correct option in the list, so this cannot be it. The Luni is the classic Indian example of a river with fresh water in the upper course and saline water in the lower.

Concept: Rivers become salty in two quite different ways, and this question is about the less obvious one. In an estuary the sea pushes salt water upstream, so the salinity is tidal and comes from outside. In an inland or endorheic basin like western Rajasthan there is no sea to reach: the river simply loses water to evaporation faster than it gains it, dissolves salts out of the desert soils and saline groundwater it crosses, and ends in a salt marsh instead of a mouth. The Luni's salinity is of this second, evaporative type — which is also why the Thar carries salt lakes such as Sambhar, Didwana and Pachpadra.

State PCS 2019 Q49 — Geography PYQ with Answer

State PCS 2019 Q49 — Geography PYQ with Answer