Erosion of the Alborz Mountains Constrained by River Gauging

The Alborz Mountains of north Iran are an active fold and thrust belt within the Arabia-Eurasia collision zone, located at the south edge of South Caspian Basin. About 36% of the continental convergence in this region is accommodated in the mountain belt. There is a strong climate asymmetry across the range; the north flank receives ten times more precipitation than the arid south flank, making this area ideal site to study climate control on erosion and crustal deformation. Suspended sediment records have been used to calculate erosion rates in the Alborz Mountains. 286 hydrometric stations in the range have an average period of record of 12 years of suspended load and water discharge. Of these, 144 stations have reported >100 suspended load measurements. For these stations, we have calculated erosion rates using mean sediment loads, converted for non-uniform sampling intervals, seasonal rating curves of discharge-suspended load and time series analysis. Sediment accumulation rates in reservoirs are employed to validate our decadal erosion rate estimates, and to constrain bedload contribution. The total suspended sediment discharge from the mountain belt is 28.5 × 10⁶ t.y^-1, giving an average erosion rate of 0.16mm.y^-1 for the Alborz Mountains. Erosion is not uniformly distributed; 40.6% of sediment discharge comes from just 5.9% of the area. Erosion rates are highest in the east and west and lower in the central block. Notably, no significant contrast exists between erosion rates in north and south flank. Our findings imply that climate has not been an important control on decadal erosion in the Alborz Mountains; the average annual sediment yield from the wet north flank and dry south flank are 7.8 × 10⁶ t.y^-1 and 20.7 × 10⁶ t.y^-1 respectively. We attribute localized, fast erosion to recent seismicity.