1. Introduction
1.1 Motivation
Advances in tectonic geomorphology require quantitative understanding
about relationships among climate, tectonics, and erosion. In temperate
mountain landscapes, studies of bedrock rivers provide important
insights into interactions between these processes (e.g., D’Arcy &
Whittaker, 2014; Harel et al., 2016; Kirby & Whipple, 2012; Lague,
2014; Olen et al., 2016; Scherler et al., 2017; Whipple & Tucker, 1999;
Whittaker, 2012). However, despite longstanding theoretical support for
the notion that climate, like tectonics, has a fundamental role in
influencing erosion (e.g., Bonnet & Crave, 2003; Howard & Kerby, 1983;
Lague, 2014; Molnar, 2001; Perron, 2017; Rinaldo et al., 1995; Tucker &
Slingerland, 1997), a general relationship between climate and erosion
has proven elusive (Perron, 2017; Whittaker, 2012). Here, we explore the
extent to which this conundrum may reflect limitations in the current
framework describing how climate-related signals should be expressed in
landscapes that, in turn, may impede recognition of diagnostic
characteristics of landscape response to climate change.
Orographic precipitation patterns are ubiquitous in mountain landscapes.
In general, they develop from the interaction of humid air masses with
topographic relief and can create dramatic spatial and elevation
dependent gradients in precipitation (see Roe, 2005 for an overview).
For instance, the Olympic, Sierra Nevada, and Wasatch ranges in western
North America all experience orographically enhanced precipitation with
increasing elevation (e.g., Barros & Lettenmaier, 1994; Barstad &
Smith, 2005; Roe, 2005). Alternatively, large tracts along eastern and
southern flanks of the Andes and Himalaya, respectively, become more
arid as elevation increases (Anders et al., 2006; Bookhagen & Burbank,
2010; Bookhagen & Strecker, 2008; Burbank et al., 2003). While numerous
factors affect orographic precipitation patterns in detail, broadly
speaking, atmospheric moisture content, topographic characteristics
(e.g., relief), and general circulation patterns are primary physical
controls on their development (e.g., Held & Soden, 2006; Roe, 2005; Roe
et al., 2008; Trenberth et al., 2003). Because atmospheric moisture
content depends strongly on temperature (i.e., Clausius-Clapeyron
relationship; Held & Soden, 2006; Roe, 2005; Trenberth et al., 2003),
shifts in temperature that accompany changes in climate must influence
these precipitation patterns (e.g., Mutz et al., 2018; Roe & Baker,
2006; Siler & Roe, 2014). Therefore, if erosional processes in these
landscapes are generally sensitive to spatial and/or temporal variations
in precipitation, then changing characteristics of orographic
precipitation patterns with changes in climate should importantly
influence mountain landscape evolution.
In mountainous settings, transverse rivers tend to cross orographic
precipitation gradients, which are generally oriented orthogonally to
the topographic trend of the range. Their tributaries, on the other
hand, typically experience a relatively muted range in precipitation due
to their orientation and/or smaller areal extent. Consequently, rivers
of different size, orientation, and position often experience
dramatically different precipitation conditions. Within large river
basins, these differences may be substantial. Exposure to orographic
precipitation patterns is expected to systematically affect river
profile concavity; increases in precipitation with distance upstream
lowers profile concavity, while the opposite trend increases profile
concavity (Han et al., 2014, 2015; Roe et al., 2002, 2003; Ward &
Galewsky, 2014). Changes in concavity driven by temporal changes in
orographic precipitation patterns require longitudinally variable
amounts of incision. Furthermore, because transverse rivers set
erosional base level for their tributaries, any along-stream variation
in incision exhibited by transverse rivers during this adjustment will
necessarily drive spatially and temporally variable base level histories
for tributaries. Developing a framework that accommodates such
variability and its influence on river profile evolution is a
fundamental need.