CSIR NET Dec 2015 Q112: Sedimentology

A graded river is one in which the longitudinal profile has adjusted so that the stream, over time, maintains a delicate balance between sediment transport capacity and sediment supply -- the classic 'graded' condition of equilibrium described by Mackin (1948). In such a river, even though channel slope generally decreases downstream, this is compensated by a corresponding increase in discharge (and often channel width/depth adjustments), such that the stream power (rate of energy expenditure per unit channel length, proportional to discharge x slope) remains fairly constant along the length of the graded river course, reflecting the self-adjusting equilibrium nature of the graded profile. This matches option 3.

Option 1 (decreases downstream) would be expected if only slope were considered without accounting for the compensating increase in discharge, but this ignores the essential self-adjustment that defines a graded river. Option 2 (increases downstream) would occur if discharge effects dominated without any moderating decrease in slope, again inconsistent with the graded equilibrium concept. Option 3 is correct: the defining characteristic of a graded river is that the profile has adjusted precisely so that stream power (and hence erosive/transport capacity relative to sediment load) remains roughly constant along its course. Option 4 (increases at tributary junctions) describes local discharge increases at confluences, but this is not the general characteristic property of a graded river's power distribution being asked about here.