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Presented at the NABS Annual meeting, Vancouver, British Columbia, 2004
in Large Rivers 1
Expansion and contraction dynamics of a large Alpine river (Tagliamento River, Italy)
U. Uehlinger, D. Schlaepfer, A. Rotach, M. Döring, and K. Tockner. Department of Limnology, EAWAG, CH-8600 Dübendorf, Switzerland
Floodplain rivers are pulsing ecosystems that expand and contract with changes in flow. We studied large scale patterns of surface drying and rewetting in a 35 km long braided section of a large Alpine gravel-bed river. We surveyed the longitudinal extent of the wetted channels using differential GPS and measured discharge and vertical hydraulic gradients to characterize vertical water exchange. During summer 2003, between 3 and 22 km of the river fell dry. Frequent irregular flow pulses caused by storms rapidly increased the length of the wetted channel (up to 3 km/h); the subsequent contraction was much slower (<0.5 km/h). In the upper part of river section (losing zone), flow decreased by 3 m3/s per river km. In the gaining (outwelling) zone, flow increased by 0.3 m3/s per km only. A high magnitude spate in September, which temporarily connected the permanent reaches, clogged surface bed sediments (reduction of the infiltration rate) and, thus, increased the length of the wetted reach. Our results show that in this low gradient flood plain of a gravel-bed river the availability of aquatic habitats was highly sensitive to changes in flow and vertical water infiltration.
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