Adjustment of a drainage network to capture induced base-level change: an example from the Sorbas Basin, SE Spain

Quaternary catchments in the south of the Sorbas Basin, SE Spain have been affected by two regionally significant river captures. The river captures w ere triggered by changes in regional gradients associated with sustained Qu aternary uplift in the region of 160 m Ma(-1). The first capture occurred i n the early Pleistocene and re-routed 15% of the original Sorbas Basin drai nage into the Carboneras Basin to the south. The second occurred in the lat e Pleistocene and re-routed 73% of the original Sorbas Basin drainage to th e east. This latter capture had dramatic consequences for base-level in the Sorbas Basin master drainage. Local base-level was lowered by 90 m at the capture site, 50 m at 7 km upstream and 25 m at 13 km upstream of the site. The base-level change instigated a complex re-organisation of the drainage networks in systems tributary to the master drainage over the ensuing peri od (some 100 ka). After the capture, drainage systems closer to the capture site experienced a tenfold increase in incision rates over most of their n etwork. Those located some 13 km upstream of the capture site experienced a fivefold increase in incision, although in this instance, the changes do n ot appear to have propagated to the headwater regions of the drainage nets. The sensitivity of individual catchments was largely governed by geologica l controls (structure and lithology). The detailed network evolution in the most sensitive areas can be traced by reconstructing former drainage pathw ays using abandoned drainage cola and the alignment and degree of incision of the drainage networks. Three main stages of evolution can be identified which record the progressive spread of base-level changes from the master d rainage. These are Stage 1 (pre-capture): original south-to-north consequen t drainage; Stage 2 (early stage, post capture): aggressive subsequent sout hwest-to-northeast and east-west drainage developed along structural lineam ents first in the east of the area (Stage 2a), and later in the west of the area (Stage 2b); and Stage 3 (late stage, post capture): obsequent drainag e developing on the topography of the Stage 2 drainage. All stages of the n etwork evolution are associated with drainage re-routing as a function of r iver capture at a variety of scales. The results highlight the complex resp onse of the fluvial system, and the Very different geomorphological histori es of adjacent catchments, emphasising the need for regional approaches for examining long-term changes in fluvial systems. (C) 2000 Elsevier Science B.V. All rights reserved.