Abstract. Landscape evolution models can be used to assess the impact of
rainfall variability on bedrock river incision over millennial timescales.
However, isolating the role of rainfall variability remains difficult in
natural environments, in part because environmental controls on river
incision such as lithological heterogeneity are poorly constrained. In this
study, we explore spatial differences in the rate of bedrock river incision
in the Ecuadorian Andes using three different stream power models. A
pronounced rainfall gradient due to orographic precipitation and high
lithological heterogeneity enable us to explore the relative roles of
these controls. First, we use an area-based stream power model to scrutinize
the role of lithological heterogeneity in river incision rates. We show that
lithological heterogeneity is key to predicting the spatial patterns of incision
rates. Accounting for lithological heterogeneity reveals a nonlinear
relationship between river steepness, a proxy for river incision, and
denudation rates derived from cosmogenic radionuclide (CRNs). Second, we explore
this nonlinearity using runoff-based and stochastic-threshold stream power
models, combined with a hydrological dataset, to calculate spatial and
temporal runoff variability. Statistical modeling suggests that the
nonlinear relationship between river steepness and denudation rates can be
attributed to a spatial runoff gradient and incision thresholds. Our
findings have two main implications for the overall interpretation of
CRN-derived denudation rates and the use of river incision models: (i)
applying sophisticated stream power models to explain denudation rates at
the landscape scale is only relevant when accounting for the confounding
role of environmental factors such as lithology, and (ii) spatial patterns in
runoff due to orographic precipitation in combination with incision
thresholds explain part of the nonlinearity between river steepness and
CRN-derived denudation rates. Our methodology can be used as a framework to
study the coupling between river incision, lithological heterogeneity and
climate at regional to continental scales.
Role of spatial patterns in fracture of disordered multiphase materials