Fluvial morphology is affected by a wide range of forcing factors, which can be external, such as faulting and changes in climate, or internal, such as variations in rock hardness or degree of fracturing. It is a challenge to separate internal and external forcing factors when they are co-located or occur coevally. Failure to account for both factors leads to potential misinterpretations. For example, steepening of a channel network due to lithologic contrasts could be misinterpreted as a function of increased tectonic displacements. These misinterpretations are enhanced over large areas, where landscape properties needed to calculate channel steepness (\textit{e.g.} channel concavity) can vary significantly in space. In this study, we investigate relative channel steepness over the Eastern Carpathians, where it has been proposed that active rock uplift in the Southeastern Carpathians gives way N- and NW-wards to ca. 8 Myrs of post-orogenic quiescence. We develop a technique to quantify relative channel steepness based on a wide range of concavities, and show that the main signal shows an increase in channel steepness from east to west across the range. Rock hardness measurements and geological studies suggest this difference is driven by lithology. When we isolate channel steepness by lithology to test for ongoing rock uplift along the range, we find steeper channels in the south of the study area compared to the same units in the North. This supports interpretations from longer timescale geological data that active rock uplift is fastest in the southern Southeastern Carpathians.

The concavity index, $\theta$, describes how quickly river channel gradient declines downstream. It is used in calculations of normalized channel steepness index, $k_{sn}$, a metric for comparing the relative steepness of channels with different drainage area. It is also used in calculating a transformed longitudinal coordinate, $\chi$, which has been employed to search for migrating drainage divides. Here we quantify the variability in $\theta$ across multiple landscapes distributed across the globe. We describe the degree to which both the spatial distribution and magnitude of $k_{sn}$ and $\chi$ can be distorted if $\theta$ is assumed, not constrained. Differences between constrained and assumed $\theta$ of 0.1 or less are unlikely to affect the spatial distribution and relative magnitude of $k_{sn}$ values, but larger differences can change the spatial distribution of $k_{sn}$ and in extreme cases invert differences in relative steepness: relatively steep areas can appear relatively gentle areas as quantified by $k_{sn}$. These inversions are function of the range of drainage area in the considered watersheds. We also demonstrate that the $\chi$ coordinate, and therefore the detection of migrating drainage divides, is sensitive to varying values of $\theta$. The median of most likely $\theta$ across a wide range of mountainous and upland environments is 0.425, with first and third quartile values of 0.225 and 0.575. This wide range of variability suggests workers should not assume any value for $\theta$, but should instead calculate a representative $\theta$ for the landscape of interest, and exclude basins for which this value is a poor fit.