In mountain areas affected by uplift, significant reorganization of drainage networks can occur through river capture. This modification can dramatically affect sediment flux and routing into adjacent sedimentary basins. It is thus important to obtain information on rates and direction of changes in catchment areas in such environments. This can be achieved where the original, precapture catchments can be compared with the postcapture scenario. This paper examines three modern mountain catchments from southeast Spain that were affected by river capture, and that fed relict Pliocene–Pleistocene alluvial fan systems. Morphometric data have been collated for complete Quaternary mountain catchment piedmont fan systems. These data are used to establish regressions to determine catchment area and basin relief from a combination of fan characteristics (area and gradient). These relationships are then used to predict the original catchment characteristics for the three Pliocene–Pleistocene relict fans and compared with the modern catchments to quantify the magnitude and direction of changes, which have affected the drainage basins since fan abandonment in the early Pleistocene. The figures suggest that the catchment areas are responding to regional uplift, most significantly by drainage net expansion facilitated by river capture rather than lowering of surface relief. As a result, sediment routing into the sedimentary basins has been radically altered.