ALBERT

Description: The Aguas/Feos river system of the Sorbas Basin, SE Spain was captured by an aggressive subsequent stream c. 100 ka. The consequence of the capture event was twofold: (1) basin-scale drainage reorganization via beheading of the southward flowing Aguas/Feos system and re-routing the drainage eastwards into the Vera Basin; and (2) the creation of a new, lower base level and associated upstream propagation of a wave of incision. The sequence of pre- and post-capture events are well established from previous studies of the Quaternary terrace record. Using these studies, this paper makes the first attempt to quantify the impact of river capture in terms of spatial and temporal variations in rates of incision, sediment flux and surface lowering. This was carried out through construction of 43 valley cross-sections from the captured' (Upper Aguas), beheaded' (Feos) and capturing' streams (Lower Aguas) within the central-southern parts of the Sorbas Basin. Dated pre- and post-capture terrace and corresponding strath levels were plotted on to the valley cross-sections enabling incision amounts, rates and valley cross-sectional areas to be calculated. Sediment fluxes were calculated using a mean valley section method. Surface lowering calculations were made through reconstruction of the top basin-fill surface and subtraction from the modern contour values. The lowering of base level has resulted in a dramatic increase in incision upstream of the capture site by a factor of 4 to 20. This in turn has been associated with significant pre- and post-capture changes in valley shape. The increased incision resulted in dramatic post-capture increases in valley erosion upstream and downstream of the capture site by a factor of 2 to 9 which can be related to changes in associated stream power as a function of increased gradient and discharge. In excess of 60% of the landscape change can be accounted for by valley-constrained erosion as opposed to overall surface lowering.