Geography Reference
In-Depth Information
to be lood-dominant, whereas those that display an ebbing tide
that is greatest are ebb-dominant. Flood- or ebb-dominance influ-
ences whether net sediment transport is landward or seaward,
respectively. Flood-dominant estuaries infill their entrance channels
by continually pushing coastal sediment landward, often causing
the exit to be clogged up, and therefore the estuary mouth changes
which makes their use for shipping difficult. Ebb-dominant estuar-
ies tend to flush sediment seawards and are more stable environ-
ments for shipping.
The outer zone in most estuaries is devoid of vegetation due to
excessive sediment and water movement. Further within the
estuary, however, salt tolerant plants can grow. In tropical environ-
ments this leads to mangroves and in temperate environments to
salt marshes. These significantly enhance sediment deposition. The
steady supply of organic matter from the plants also adds to the
sediment, which means that these zones rise in altitude over time
as the sediment accumulates. This build-up of sediment is often at
a faster rate than sea level rise meaning that these environments can
rise at the same rate as sea levels, as long as they are protected from
human damage.
Deltas are coastal landforms dominated by river processes.
They are accumulations of sediment deposited where rivers enter
into the sea. Here the amount of sediment delivered by the river
is greater than that removed by waves and tides and so the delta
moves seawards. Lots of people live on deltas, although there are
not many deltas around the world as they tend only to be associ-
ated with large river systems (e.g. Mississippi, Ganges, Lena, Nile).
The coarsest sediments are deposited close to the river mouth and
the finer sediments settle out further seaward. Ongoing delta sur-
vival and development relies on an active sediment supply by the
river. Deltas can be very dynamic. As one area of the delta grows
upwards due to building sediment, the river channels can suddenly
no longer keep flowing to that raised area and so they move across
the delta supplying sediment elsewhere. The starving of sediment
from some areas of delta can then lead to net erosion through
wave and tide action. This dynamism creates a hazard for humans
living on deltas. Humans can also modify the sediment supply in a
river, by building dams for example, which can lead to delta
erosion.
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