What is the main geographical reason for deep gorges in the upper courses of the Indus and Brahmaputra?

Correct answer: Strong vertical erosion and mountain uplift

The direct answer is C: strong vertical erosion and mountain uplift. In the upper courses, Himalayan rivers flow down steep gradients through high mountains. Rapid flow has great energy, especially for cutting the river bed downward. At the same time, tectonic uplift raises the land and keeps the slope and relief high. The river therefore cuts deeply instead of spreading widely, producing narrow, deep gorges. Option A is wrong because low slope and slow current reduce erosive power and favour deposition. Option B is wrong because sea tides do not control these high mountain valleys far inland. Option C is correct because active uplift maintains relief while vertical erosion deepens the valleys. Option D is wrong because desert wind deposition forms wind-blown deposits, not river-cut gorges. Exam cue: a steep youthful river plus uplift usually means vertical down-cutting and gorges.