The impact of river capture on fluvial terraces and bedrock incision

Abstract

River terraces are commonly used to infer climate and tectonic histories. Yet, it is increasingly recognised that other processes, such as river capture, can affect river terrace genesis and incision rates and patterns. In this study, we conduct a field-based investigation of river terrace sequences along the Kolokithas and Varitis Rivers in central Crete, Greece, that share a confluence and preserve geomorphic evidence for the recent capture of the Kolokithas headwaters by the Varitis. We use digital topographic analysis, mapping, and optically stimulated luminescence (OSL) geochronology to quantify the river terrace and bedrock incision response to river capture. Topographic analysis indicates the Varitis captured ~30 km² of drainage area from the Kolokithas. We find differences in terrace characteristics, number of terraces, and incision rates and patterns on the adjacent valleys. The Kolokithas has four terrace levels, and the Varitis has five. All terraces are strath terraces, except for the oldest on the Kolokithas, a ~8 m thick fill terrace that starkly contrasts the time-equivalent ~1–2 m thick strath terrace on the Varitis. Relative and absolute age control suggests three Pleistocene terraces were emplaced during cooler climate intervals, and two Holocene terraces are perhaps because of anthropogenic disturbances. The incision patterns differ on each valley, with generally more incision upstream on the Varitis relative to the Kolokithas. Incision rates on the Varitis are roughly twice as high as on the Kolokithas, but the average incision rate of both valleys combined is comparable to coastal rock uplift rates derived from marine terraces. Collectively, our results suggest that fluvial systems are sensitive to climate and tectonic processes even when affected by geomorphic disturbances, like river capture and beheading. However, care must be taken when interpreting river terraces as direct records of climate and tectonic processes, particularly when working on a single river valley.