Geoscience Reference
In-Depth Information
Similar links between sediment sources and
sinks are evident along many coastlines where
sediment is supplied either from fluvial sources
or from one area of coastline to another, often
via longshore transport. Reductions in sediment
supply often lead to increased rates of erosion
along the coastal sectors that are deprived of
sediment. This occurs, for example, where sedi-
ments are trapped behind dams located on the
rivers that feed the coastal sector. In California,
reduced fluvial sediment supply due to damming
has led to increased rates of cliff erosion (see
Chapter 8), and extensive subsidence and shore-
line erosion of the Nile delta is attributed to
significantly reduced sediment supply to the
delta front (see Chapter 7). In the northern Gulf
of California fundamental changes in the sources
and rates of sediment supply, and in the composi-
tion of accumulating sediment, have also been
directly linked to the effects of dam construction
and have resulted in a 95% reduction in sedi-
ment supply from the Colorado River (see Case
Study 1.1). Coastal retreat may also occur where
'upstream' sediment supply has been restricted
by sea-defence construction. In such cases, sea-
wall or groyne systems either prevent or restrict
sediment throughput, leading to increased rates
of downstream beach erosion and shoreline
change (see Chapter 8). The recognition of these
important sedimentary links has been a key
driver in the development of integrated catch-
ment and coastal management schemes.
Anthropogenic-related modifications to sedi-
ment source and transport pathways may be
significantly exacerbated by the effects of urban-
ization. In relation to sedimentary systems, the
most important influence occurs where construc-
tional activities occur within the sedimentologic-
ally active zone. Problems arise either because of
construction in areas where episodic sediment-
ological and geomorphological changes can be
expected, or where deliberate constructional act-
ivities have a consequent effect upon pathways
of sediment transport and zones of sediment
accumulation. Coastal dune and barrier island
sequences, for example, form part of active sedi-
mentary systems that will respond to high-energy
storm events. Hence, roads and houses built in
such zones become susceptible to storm damage
(see Chapters 8 and 9). Similarly, construction
of seawalls or other landward constraints may
lead to 'coastal squeeze' as landward migration
Case study 1.1 Anthropogenic modifications to sediment supply, northern Gulf of California
The Gulf of California is a narrow epicontinental sea, 1500 km long, that has formed from
tectonic activity along the Californian coast (Case Fig. 1.1). The Northern Gulf of California
(NGC) receives sediment from four areas, the Colorado River, the batholith of the Baja California,
the Sierra Madre Occidental, and the deserts of north-west Mexico. Each of these source areas
has a distinctive mineralogical signature enabling the provenance of accumulating marine sedi-
ments in the NGC to be determined. On this basis four distinct sediment provinces have been
identified, these being, (i) the Colorado River Delta Province, (ii) the Concepción River Province,
(iii) the Transitional Province and (iv) the Baja-Sonora Province (Case Fig. 1.1).
Historically, the Colorado River has been the primary source of sediment into the NGC, with
an estimated annual sediment discharge of 160
10
6
t. Fluvial sediment supply has, however,
been dramatically reduced over the past 100 years following the construction of a series of
dams along the river. Of particular significance have been the Hoover Dam (built in 1934) and
the Glenn Canyon Dam (built in 1952). The result of this extensive water flow regulation has
been to reduce fluvial sediment supply by around 95%, resulting in sediment starvation to both
estuarine and deltaic environments of the Colorado River mouth, and in the northern areas of
the Gulf.
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