Geomorphic Heterogeneity Across the Chalus–Karaj Drainage Divide in the Central Alborz, Iran

ABSTRACT

Geomorphic contrasts across drainage divides can reflect several interacting controls. We examine how contrasts across the shared Chalus–Karaj divide in the Central Alborz, Iran, vary among metrics and explicitly defined spatial populations. We combine basin morphometry, selected channel‐network diagnostics, fixed‐reference‐area topography, segmented Mχ as a relative steepness proxy and route‐based knickpoint associations with mapped faults and lithologic contacts. In the selected analytical networks, mean Mχ is higher in Chalus (184.35 vs. 129.73), and knickpoint densities are 2.903 versus 1.871 km ⁻¹ . At Aref = 1.0 km ² , the primary 15‐endpoint population gives Chalus‐minus‐Karaj contrasts of +102.61 m for channel elevation, −0.0630 for mean upstream gradient and −43.02 m for R500. Four locally co‐supported segments cover 12.45% of the divide; their R500 contrasts range from −76.31 to +92.55 m. A separate raster‐based selection gives 19 catchments and a pooled R500 contrast of −26.26 m. For the same fitted Mχ field, primary upstream endpoint contrasts are +4.21 under equal‐node weighting and −4.88 under equal‐source weighting; a 17‐endpoint connector sensitivity gives +2.04 under equal‐source weighting. Route‐level fault and contact associations are modest and sensitivity dependent. These results support spatial and metric‐dependent geomorphic heterogeneity, with additional sensitivity to sampling population and averaging weights. They do not isolate an independent scale effect, a unique causal mechanism or a regional drainage‐divide migration direction or rate.