Orographic rainfall patterns are ubiquitous features of mountain ranges that strongly affect erosional processes and topography, and accounting for their effect is critical when evaluating the processes driving a landscape’s evolution. River drainage piracy – or stream capture – is one such process that is currently receiving considerable attention because it can drive species integration, influence erosional patterns on short-term to million-year timescales, and potentially induce tectonic responses to focused erosional unloading, among other exciting possibilities. Importantly, however, neglecting the effect of orographic rainfall patterns produces morphometric signatures in standard analyses that precisely mirror those expected from river piracy, thus creating the potential to mistake climate-driven topographic features for river piracy events. Here, we present a detailed river profile analysis of the Arun River and surrounding catchments in the eastern Nepal Himalaya that exemplifies how artifactual features created by neglecting the effect of orographic rainfall patterns can mirror those of stream network disequilibrium and river piracy. Specifically, we show that across-divide χ anomalies, contrasts between transverse river and tributary channel steepness (k_sn), and elevated k_sn along the Arun River compared to transverse rivers of surrounding catchments – all expected features from a recent sudden increase in drainage area – are eliminated by using rainfall-weighted morphometric variants χ_Q and k_snQ. This result suggests that all these features are likely artifacts of the assumption imbedded into standard drainage-area weighted metrics χ and k_sn that rainfall is uniformly distributed. Additionally, whereas standard interpretations of χ and k_sn patterns suggest recent capture-induced stream network disequilibrium and ongoing focused incision, rainfall-weighted χ_Q and k_snQ imply widespread first-order adjustment to the strong orographic rainfall pattern across this region and to non-uniformities in other factors such as rock uplift rate and bedrock erodibility (i.e., quasi-equilibrium conditions), an alternative further supported by the regional geomorphology. While rainfall-weighted metrics χ_Q and k_snQ also imbed an assumption that mean-annual rainfall patterns scale with geomorphically relevant floods, which is likely not true in some cases, this study adds evidence that such exceptions may be rarer – or to a lesser degree – than is perhaps widely assumed. Together, our results underscore that accounting for orographic rainfall patterns is essential for understanding mountain landscape evolution, particularly when assessing stream network disequilibrium and evaluating potential occurrences of river piracy.