Morphology of a tidal river subject to uplift: introducing the Kapuas (East Kalimantan, Indonesia)

The Kapuas is the largest river in Indonesia and the world's longest river on an island, stretching over more than 1.100 km in a relatively pristine region dominated by lowland forest and peatlands (Hidayat et al., 2017). The Kapuas region is subject to uplift, causing an upward convex river bed profile with a knickpoint where the river reaches a lowland plain. A two-year field programme and the subsequent analyses revealed new insights into the physical functioning of tidal rivers in a tectonic setting. The point where the bed reaches sea level effectively limits the tidal intrusion (Käster et al., 2019). Along the seaward part of the tidal river where the channel width is nearly constant, reflected tidal wave energy increases the tidal amplitudes. Along the upstream parts of the tidal river, the depth reduction caused by upward sloping of the riverbed increases the damping, so that the tide attenuates rapidly. Closest to the coast, where the channel width increases and the channel planform is funnel shaped, large-scale scour occurs. This local deepening shows similarity with scour related to drawdown during peak discharges, such as observed in the Mississippi, but the underlying mechanism is fundamentally different. Channel funneling amplifies the tide more strongly than that bed friction attenuates the tide, creating gradients in the residual sediment transport, leading to erosion. Laterally, the suspended load dominated tidal channels are stable (Kastner et al., 2017). This also holds for strongly asymmetrical channel junctions (Kästner and Hoitink, submitted). Wide and deep inlets to the side branches reduce the strength of secondary circulation, preventing the formation of a plug bar in the side channel. The planform stability in the lowland plain is in stark contrast with the meandering activity upstream of the concave section of the river, where an inland plain captures sediment and the river is hyper-morphodynamic. Hidayat, H., Teuling, A. J., Vermeulen, B., Taufik, M., Kästner, K., Geertsema, T. J., Bol, D. C. C., Hoekman, D. H., Haryani, G. S., Van Lanen, H. A. J., Delinom, R. M., Dijksma, R. Anshari, G. Z. Ningsih, N. S., Uijlenhoet, R. and Hoitink, A. J. F. (2017). Hydrology of inland tropical lowlands: The Kapuas and Mahakam wetlands. Hydrology and earth system sciences, 21(5), 2579. Kästner, K., Hoitink, A. J. F., Torfs, P. J. J. F., Deleersnijder, E., & Ningsih, N. S. (2019). Propagation of tides along a river with a sloping bed. Journal of Fluid Mechanics, 872, 39-73. Kästner, K., Hoitink, A. J. F., Vermeulen, B., Geertsema, T. J., & Ningsih, N. S. (2017). Distributary channels in the fluvial to tidal transition zone. Journal of Geophysical Research: Earth Surface, 122(3), 696-710. Kästner, K. & Hoitink, A. J. F. (2019). Flow and suspended sediment division at two asymmetric channel bifurcations in a river delta: implications for channel stability. Submitted to Journal of Geophysical Research: Earth Surface