jats:titleAbstract</jats:title>jats:pA range of complex hydraulic and geomorphic processes shape terrestrial landscapes. It remains unclear how these processes act to generate observed drainage networks across scales of interest. To address this issue, we transform observed and synthetic longitudinal river profiles into the spectral domain with a view to interrogating the different scales at which fluvial landscapes are generated. North American river profiles are characterized by red noise (i.e., spectral power, jats:italicϕ</jats:italic> ∝ jats:italick</jats:italic>jats:sup−2</jats:sup>, where jats:italick</jats:italic> is wave number) at wavelengths >100 km and pink noise (jats:italicϕ</jats:italic> ∝ jats:italick</jats:italic>jats:sup−1</jats:sup>) at shorter wavelengths. This observation suggests that river profile geometries are scale‐dependent and using small‐scale observations to develop a general understanding of large‐scale landscape evolution is not straightforward. At wavelengths >100 km, river profile geometries appear to be controlled by smoothly varying patterns of regional uplift and slope‐dependent incision. Landscape simulations, based upon stream power that are externally forced by regional uplift do not exhibit a spectral transition from red to pink noise because these simulations do not incorporate heterogeneous erodibility. Spectral analysis of erodibility extracted from patterns of lithologic variation along river profiles suggests that the missing spectral transition is accounted for by heterogeneous substrates, which are characterized by white or blue noise (jats:italicϕ</jats:italic> ∝ jats:italick</jats:italic>jats:sup0</jats:sup> or jats:italick</jats:italic>jats:sup1</jats:sup>). Our results have implications for the way by which rivers record large‐scale tectonic forcing while incising through complex lithologic patterns.</jats:p>