The problem of underpowered rivers

Abstract

This study has hypothesized that for many rivers the trade‐off between flow accumulation and the decrease in slope along channel length means that stream power increases downstream and, moreover, that given the low slope angles in headwater and low‐order streams, they would have insufficient stream power to erode let alone transport sediment. The study considered the stream power profile, the particle travel distances and the application of the Hjulström curve based on the velocity profile of nine, large UK catchments. The study showed that: Some rivers never showed a maximum in their longitudinal stream power profile, implying that some rivers never develop a deposition zone before they discharge at the tidal limit. Particle travel distances during a bankfull discharge event showed that for some rivers 91% of the upper main channel would not be cleared of sediment. Furthermore, while some rivers could transport a 2 mm particle their entire length in one bankfull event, for another river it would take 89 such events. The Hjulström curve shows that for three of the study rivers the upper 20 km of the river was not capable of eroding a 2 μm particle. The study has shown that for all rivers studied, erosion is focused downstream and deposition upstream. Many UK rivers have a dead zone where, on time scales of the order of centuries, no erosion or transport occurs and erosion only occurs in the lower courses of the channel where discharge rather than slope dominates – we propose these as underpowered rivers.