Abstract
Coastal lowland plains under mixed fluvial‐tidal influence may form complex, composite channel networks, where distributaries blend the characteristics of mouth bar channels, avulsion channels, and tidal creeks. The Kapuas coastal plain exemplifies such a coastal plain, where several narrow distributaries branch off the Kapuas River at highly asymmetric bifurcations. A comprehensive geomorphological analysis shows that trends in the channel geometry of all Kapuas distributaries are similar. They consist of a short, converging reach near the sea and a nonconverging reach upstream. The two parts are separated by a clear break in scaling of geometrical properties. Such a break in scaling was previously established in the Mahakam Delta, which suggests that this may be a general characteristic in the fluvial to tidal transition zone. In contrast to the geometrical trend similarities, a clear difference in bed material between the main and side distributaries is found. In the main distributary, a continuous trend of downstream fining is established, similar to what is often found in lowland rivers. In the side distributaries, bed material coarsens in the downstream direction. This indicates an undersupply of sediment to the side distributaries, which may contribute to their long‐term stability as established from historical maps. Tides may be the main agent preventing fine sediment to settle, promoting residual transport of fine material to the coastal ocean.
Highlights
River deltas are formed by deposition and reworking of sediment [Davis Jr. and Dalrymple, 2011]
All delta branches terminate in shallow mouth bars, where the bed almost emerges at low tide
Consistent with downstream fining in the main distributary, the initial grain sizes in the side distributaries directly after branching off are lower when the bifurcation is located further downstream
Summary
River deltas are formed by deposition and reworking of sediment [Davis Jr. and Dalrymple, 2011]. The Mahakam forms a typical mixed river tide-dominated delta, with tidal channels often disconnected from fluvial distributaries (Figure 1). Sassi et al [2012] revealed a break in cross-section geometrical properties in the Mahakam distributaries, which separates tide-dominated reaches downstream from the river-dominated reaches further upstream. Our aim is to better understand downstream trends in channel geometrical properties and bed material grain size across the fluvial to tidal transition zone by focusing on the Kapuas lowland plain. Individual distributaries blend the properties of mouth bar channels, avulsion channels, and tidal creeks. These three channel types are defined in greater detail hereafter
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