KvR report 001/2006are <strong>the</strong> Fraser delta in Canada (2647 people km -2 ) and <strong>the</strong> Chao Praya in Thailand(1763 people km -2 ).<str<strong>on</strong>g>Deltas</str<strong>on</strong>g> are also important areas in ec<strong>on</strong>omic terms. Many <strong>of</strong> <strong>the</strong> world’s major portsare situated at <strong>the</strong> mouths <strong>of</strong> delta-forming rivers, including Rotterdam andShanghai. The proximity <strong>of</strong> transport routes over <strong>the</strong> water, where <strong>the</strong> sea and <strong>the</strong>river are directly c<strong>on</strong>nected <strong>with</strong> each o<strong>the</strong>r, makes such a site ideal for trade andindustry. Products, and <strong>the</strong>refore, m<strong>on</strong>ey traditi<strong>on</strong>ally flows into countries through<strong>the</strong>ir ports. Rotterdam harbour in <strong>the</strong> Rhine delta for example c<strong>on</strong>tributes 7,5% to <strong>the</strong>BNP <strong>of</strong> <strong>the</strong> Ne<strong>the</strong>rlands (Kamer van Koophandel Rotterdam). In many cases <strong>deltas</strong>have <strong>the</strong> fastest ec<strong>on</strong>omic growth (Figure 3.1).<str<strong>on</strong>g>Deltas</str<strong>on</strong>g> <strong>on</strong> <strong>the</strong> <strong>move</strong>The levee breakallowed water fromLake P<strong>on</strong>tchartrain t<strong>of</strong>lood New Orleans(www.Katrinahelp.com).Figure 3.1:Oil and gas reserves are <strong>of</strong>ten related to delta regi<strong>on</strong>s, for example in<strong>the</strong> Niger delta (Nigeria). Here, <strong>the</strong> delta sediment has been deposited<strong>on</strong> a much older oil-bearing sedimentary basin (Urhobo HistoricalSociety, 1996-2006). The Mississippi and <strong>the</strong> Rhine also dischargeinto oil and gas-bearing <strong>deltas</strong>.3.2 How do societies influence a delta ?In a natural delta, <strong>the</strong> supply <strong>of</strong> sediment from <strong>the</strong> land is in a dynamic equilibrium<strong>with</strong> <strong>the</strong> sea level and <strong>the</strong> erosive processes at work al<strong>on</strong>g <strong>the</strong> coast. If anythingchanges <strong>on</strong> ei<strong>the</strong>r side, <strong>the</strong> delta will find a new equilibrium. Inhabitants <strong>of</strong> <strong>deltas</strong>have been adapting <strong>the</strong>ir envir<strong>on</strong>ment for centuries to suit <strong>the</strong>ir own needs.Measures have been taken to prevent flooding (through <strong>the</strong> c<strong>on</strong>structi<strong>on</strong> <strong>of</strong> dikes,dams and delta works), to make shipping possible (by canalizing and/or dammingrivers and c<strong>on</strong>structing harbors), to enable farming (by cutting down woods andleveling and draining <strong>the</strong> land), and to allow <strong>the</strong> extracti<strong>on</strong> <strong>of</strong> natural recourses(sand, clay, peat).Human interventi<strong>on</strong> has generally led to <strong>the</strong> disrupti<strong>on</strong> or complete obstructi<strong>on</strong> <strong>of</strong> <strong>the</strong>natural processes <strong>with</strong>in a delta. C<strong>on</strong>sequently, a delta can lose its natural flexibilityand is no l<strong>on</strong>ger able to adapt to changing circumstances. For example, <strong>the</strong> removal<strong>of</strong> mangroves and <strong>the</strong> damming <strong>of</strong> major rivers for <strong>the</strong> benefit <strong>of</strong> shrimp farms hasresulted in large-scale erosi<strong>on</strong> <strong>of</strong> <strong>the</strong> coastline <strong>of</strong> Thailand (Thampanya et al., 2006).As human populati<strong>on</strong> increases in coastal areas, so does pressure <strong>on</strong> coastal anddeltaic ecosystems through habitat c<strong>on</strong>versi<strong>on</strong>, increased polluti<strong>on</strong>, and demand forcoastal resources. The more people that crowd into coastal areas, <strong>the</strong> more pressure<strong>the</strong>y impose both <strong>on</strong> land and sea. Natural landscapes and habitats are altered,overwhelmed and destroyed to accommodate <strong>the</strong>m. Lago<strong>on</strong>s and coastal waters are'reclaimed', wetlands are drained, <strong>the</strong> floodplains are built over and reduced, andmangroves and o<strong>the</strong>r forests are cut down (UN atlas <strong>of</strong> <strong>the</strong> oceans).Al<strong>on</strong>gside <strong>the</strong> many advantages, however, living and working in a delta regi<strong>on</strong> alsohas a major disadvantage. Since <strong>deltas</strong> are found in low-lying coastal areas, <strong>the</strong>y arenaturally vulnerable to all kinds <strong>of</strong> natural disasters such as flooding, storms andtsunamis. Hurricane Katrina (see photo), which raged over New Orleans last winter,dem<strong>on</strong>strated <strong>on</strong>ce again just how dangerous life in a delta can be (Hecht, 2006).The c<strong>on</strong>structi<strong>on</strong> <strong>of</strong> dikes in <strong>the</strong> Dutch delta, in combinati<strong>on</strong> <strong>with</strong> drainage, hasresulted in large areas no l<strong>on</strong>ger being flooded and no more sediment beingdeposited and subsidence. C<strong>on</strong>sequently, large areas <strong>of</strong> <strong>the</strong> Ne<strong>the</strong>rlands now liebelow sea level. Due to <strong>the</strong> c<strong>on</strong>structi<strong>on</strong> <strong>of</strong> dams and reservoirs in <strong>the</strong> hinterland <strong>of</strong><strong>deltas</strong>, <strong>the</strong> supply <strong>of</strong> sediment to <strong>the</strong> sea has in many cases been drasticallyreduced.The Nile delta is currently undergoing an overall transgressi<strong>on</strong> as a result <strong>of</strong>decreasing water discharge and sedimentati<strong>on</strong> rates caused by c<strong>on</strong>structi<strong>on</strong> <strong>of</strong> <strong>the</strong>Aswan Dam upstream. The use <strong>of</strong> river water for irrigati<strong>on</strong> can even result in <strong>the</strong>lower reaches <strong>of</strong> a river running completely dry, as in <strong>the</strong> case <strong>of</strong> <strong>the</strong> Colorado riverin <strong>the</strong> USA. The water <strong>of</strong> various distributaries is sometimes also brought toge<strong>the</strong>r ina single water course, for example where <strong>the</strong>re is a port. In tidal-dominated <strong>deltas</strong> inparticular, changes in <strong>the</strong> water management <strong>of</strong> <strong>the</strong> various distributaries <strong>of</strong> a deltaresults in salty seawater reaching fur<strong>the</strong>r into those distributaries where <strong>the</strong> drainage<strong>of</strong> water has decreased.2223
KvR report 001/2006<str<strong>on</strong>g>Deltas</str<strong>on</strong>g> <strong>on</strong> <strong>the</strong> <strong>move</strong>A part <strong>of</strong> <strong>the</strong> Colorado river flows into <strong>the</strong>Salt<strong>on</strong> Sea. The great demand <strong>of</strong> river waterfor irrigati<strong>on</strong> and drinking water causes alarge decrease <strong>of</strong> <strong>the</strong> river discharge, <strong>with</strong>siltati<strong>on</strong> and algal bloom (green color) in <strong>the</strong>Salt<strong>on</strong> Sea as a result. The need for freshwater from San Diego and Los Angeles is stillgrowing. System-based measures arerequired!Changes deeper underground such as <strong>the</strong> extracti<strong>on</strong> <strong>of</strong> oil, gas or salt and <strong>the</strong>pumping up <strong>of</strong> water also have <strong>effects</strong>. They ensure that <strong>the</strong> ground subsides morequickly than in a natural situati<strong>on</strong>.In <strong>the</strong> Philippines largeareas <strong>of</strong> mangroves arec<strong>on</strong>verted to fish p<strong>on</strong>ds(R<strong>on</strong> Janssen).The main effect <strong>of</strong> <strong>the</strong> majority <strong>of</strong> human interventi<strong>on</strong>s is a shortage <strong>of</strong> sediment in<strong>the</strong> delta. C<strong>on</strong>sequently, <strong>the</strong> relative influence <strong>of</strong> <strong>the</strong> sea increases, resulting inerosi<strong>on</strong> at <strong>the</strong> delta fr<strong>on</strong>t and subsidence <strong>of</strong> <strong>the</strong> delta. For inhabitants <strong>of</strong> <strong>the</strong> deltaarea, this means an increased risk <strong>of</strong> flooding. People <strong>of</strong>ten try to prevent this byc<strong>on</strong>structing dikes, resulting in areas no l<strong>on</strong>ger pr<strong>of</strong>iting from <strong>the</strong> natural build-up dueto <strong>the</strong> supply <strong>of</strong> sediment. The resulting situati<strong>on</strong> is even fur<strong>the</strong>r re<strong>move</strong>d from anatural situati<strong>on</strong>.24This pair <strong>of</strong> images shows <strong>the</strong> Three Gorges Dam in <strong>the</strong> Yangtze River in China,before and after closure. In <strong>the</strong> upper photo (July 2000) <strong>the</strong> dam is partially closed,but sediment-filled water still flowed freely al<strong>on</strong>g <strong>the</strong> river’s south bank. By May 2006,<strong>the</strong> dam spanned <strong>the</strong> entire river, and a large reservoir had already filled, in which all<strong>the</strong> sediment is captured. Clear water shoots through gates in <strong>the</strong> center porti<strong>on</strong> <strong>of</strong><strong>the</strong> dam (www.esa.int).25
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