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.