Variability of in-channel sediment storage in a river draining highly erodible areas (the Isábena, Ebro Basin)

Abstract

In-channel sediment storage is a fundamental component of a river basin’s sediment budget. Sediment remains stored until a competent flow re-suspends and transfers it downstream. The objectives of this paper are: (1) to quantify in-channel sediment storage and its spatial and temporal dynamics in the River Isabena, a mesoscale mountainous catchment draining highly erodible areas (badlands) in the south central Pyrenees (Ebro basin) and (2) to analyse changes in storage in the mainstem channel in relation to sediment yield from the main tributaries. In-channel sediment storage was measured seasonally (from winter 2011 to winter 2012) at 14 mainstem cross-sections using a re-suspension cylinder. A minimum of three locations were sampled at each section, and two levels of agitation were applied. Samples allowed determination of the amount of sediment accumulated per unit surface area at a given point in the river; estimates of the total storage in the bed of the mainstem Isabena were derived from these data. In addition, main five tributaries were monitored for discharge and suspended sediment transport. Results show an annual sedimentary cycle, with the sediment being produced in badlands during winter, transferred to the main channel during spring, stored in the river during summer and, finally, exported out of the basin by the autumn floods. Marked spatial variability was observed; sections located immediately downstream from the main tributaries (i.e. mainly Villacarli) generally held larger amounts of sediment in the bed. Runoff and sediment inputs from the tributaries were the most important factors determining sediment storage and its spatial and temporal dynamics. The overall sediment yield of the Isabena was much higher than the in-channel sediment storage, despite the large amounts stored in the channel. This finding corroborates a previous published hypothesis that fine sediment in the drainage network has a mean residence time of the order of 1 year and that the basin’s delivery ratio exceeds 90 %; both of these characteristics can be related to the high connectivity between production areas (badlands) and the river network, and to the role of baseflows allowing continuous export of sediment from the catchment.