CHAPTER 8: FLUVIAL PROCESSES, PATTERNS AND LANDFORMS
Deposition 1 Before flood
Flood stage water level Thickest and coarsest sediments
deposited at channel edges 2 During flood
Thin layers of fine alluvium deposited over outer parts of the flood plain
Natural levees contain river
3 After many floods Former channel bed Fig. 38 Exam Diagram: The development of levees
Fig. 39 Map showing levees on the River Moy, County Mayo
River bed above level of flood plain
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Fig. 40 The photographs show both sides of a levee on the banks of the River Barrow, County Kilkenny. The water level is higher than the flood plain in the photo on the right.
2. Deltas Main processes: deposition Example: Lough Tay, County Wicklow – a lacustrine delta; Shannon Estuary, County Limerick – an estuarine delta. Deltas are landforms of river deposition found in the old stage of a river. Deltas are areas of land at the mouth of a river made from deposits of alluvium, sand and gravel. Deltas occur where tidal currents and longshore drift (see Chapter 10) are not powerful enough to remove the river deposits. Deltas can form in a lake and are called lacustrine deltas; those that occur at the coast are called marine deltas. For marine deltas to form, a river has to flow out into a low energy coastline. As the Irish coastline is so indented (jagged) this does not occur. However delta deposits have built up along some Irish estuaries, e.g. the Shannon Estuary.