Read the passage and answer the questions given below it.
A river fed by ice can look its healthiest just when its future is least secure. That is the puzzle of the Himalayan glaciers. The Bhagirathi, a headstream of the Ganga, flows from the snout of the Gangotri glacier, and many northern rivers draw on ice in their upper reaches. Survey after survey finds that most Himalayan glaciers are thinning and retreating.
Retreat, oddly, does not at first mean less water. A glacier that melts faster sends more water downstream, so a river may run fuller for years while the ice that feeds it shrinks. Scientists call the turning point ‘peak water’. After it, the melt begins to fall, because less ice is left to melt. The extra flow is not a sign of health. It is a store being spent.
Nor does this matter most in the monsoon. During the rains, the plains hardly need the glaciers, since rainfall dwarfs their share. It is in the hot, dry weeks before the monsoon, when fields are parched, that meltwater attenuates the effects of the shortage. The Indus basin, with its drier lowlands, leans on this supply far more heavily than the Ganga basin, where rain carries more of the load.
Retreat brings a quicker danger too. Meltwater often pools in lakes behind loose walls of rock and debris left by the glacier. If such a wall gives way, the lake can empty within hours, and the flood that follows, a glacial lake outburst flood, tears down the valley with little warning. None of this means the rivers will soon run dry. When peak water arrives differs from basin to basin, and for some rivers no one can yet say whether it has already passed.
Which of the following best expresses the central idea of the passage?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Shrinking glaciers may swell rivers for a time, but this hides a loss felt most in dry months.
The passage is built around one puzzle: a glacier-fed river can look healthy just when its future is least secure.
- A glacier that melts faster sends more water downstream. So the river may run fuller for years while the ice shrinks. This is what swell means here: grow bigger or fuller.
- After ‘peak water’ the melt falls. The writer calls the extra flow “a store being spent”. So the gain hides a loss.
- The third paragraph says the loss matters most in “the hot, dry weeks before the monsoon”, not during the rains.
Why the Other Options are Incorrect?
Option 1: Glacial lake outburst floods have now become the gravest threat posed by the retreat of Himalayan glaciers. — Incorrect. Too narrow: The floods are one danger in the last paragraph. The passage never ranks them as the gravest, or most serious, threat.
Option 2: Rivers of the north will soon run short of water because the glaciers feeding them are fast disappearing. — Incorrect. Contradicted: The writer says, “None of this means the rivers will soon run dry.”
Option 4: The Indus leans on meltwater more heavily than the Ganga does, because its lowlands are drier. — Incorrect. Too narrow: This is true in the passage, but it is one detail in the third paragraph, not the central idea.