Large boulders with a diameter of up to several tens of meters are
globally observed in mountainous bedrock channel environments. Recent
theories suggest that high concentrations of boulders are associated
with changes in channel morphology. However, data are scarce and
ambiguous, and process-related studies are limited. Here we present data
from the Liwu River, Taiwan, showing that channel width and slope
increase with boulder concentration. We apply two mass balance
principles of bedrock erosion and sediment transport and develop a
theory to explain the steepening and widening trends. Five mechanisms
are considered and compared to the field data. The cover effect by
immobile boulders is found to have no influence on channel width.
Channel width can partially be explained by boulder control on the tools
effect and on the partitioning of the flow shear stress. However, none
of the mechanisms we explored can adequately explain the scattered width
data, potentially indicating a long-timescale adjustment of channel
width to boulder input. Steepening can be best described by assuming a
reduction of sediment transport efficiency with boulder concentration.
We find that boulders represent a significant perturbation to the
fluvial landscape. Channels tend to adjust to this perturbation leading
to a new morphology that differs from boulder-free channels. The general
approach presented here can be further expanded to explore the role of
other boulder-related processes.