Running Water - The Changing Earth: Exploring Geology and Evolution - page 310

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velocity and depth rapidly decrease. As a result, ridges of
sandy alluvium known as natural levees are deposited
along the channel margins, and mud is carried beyond
the natural levees into the floodplain where it settles from
suspension (Figure 12.13b, c).

What Would You Do

Given what is known about the dynamics of running water in
channels, it is remarkable that houses are still built on the cut
banks of meandering rivers. No doubt, property owners think
these locations provide good views because they sit high above
the adjacent channel. Explain why you would or would not build
a house in such a location. What recommendations would
you make to a planning commission on land use for areas as
described here? Are there any specifi c zoning regulations or
building codes you might favor?

Deltas
Where a river or stream flows into a standing body of
water, such as a lake or the ocean, its flow velocity rap-
idly diminishes and any sediment in transport is depos-
ited. Under some circumstances, this deposition creates a
delta, an alluvial deposit that causes the shoreline to build
outward into the lake or sea, a process called progradation.
The simplest prograding deltas have a characteristic ver-
tical sequence of bottomset beds overlain successively by
foreset beds and topset beds (

are deposited as gently inclined layers, and topset beds,
consisting of the coarsest sediments, are deposited in a net-
work of distributary channels traversing the top of the delta
(Figure 12.14).
Small deltas in lakes may have the three-part sequence
described above, but deltas deposited along seacoasts are

Figure 12.14). This vertical
sequence develops when a river or stream enters another
body of water where the finest sediment (silt and clay)
is carried some distance out into the lake or sea; there it
settles to form bottomset beds. Nearer shore, foreset beds

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Areas of maximum velocity

Bluffs

Meander neck

Cut bank Natural levees Floodplain

mud deposits

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Figure 12.10 Diagrammatic View of a Meandering Stream The length of the arrows is proportional
to fl ow velocity.