C5 p66-70 Disappearing Delta
A The fertile land of the Nile delta is being eroded along Egypt's Mediterranean coast at anastounding rate, in some parts estimated at 100 metres per year. In the past, land scoured away from the coastline by the current of the Mediterranean Sea used to be replaced by sediment brought down to the delta by the River Nile, but this is no longer happening.
14B Up to now, people have blamed this loss of delta land on the two large dams at Aswan in the south of Egypt, which hold back virtually oil of the sediment that used to flow down the river. Before the dams were built, the Nile Bowed freely, carrying huge quantities of sediment north from Africa'sinterior to be deposited on the Nile delta. This continued for 7000 years, eventually covering a region of over 22,000 square kilometres with layers of fertile silt. Annual flooding brought in new, nutrient-rich soil to the delta region, replacing what had been washed away by the sea, and dispensing with the need for fertilizers in Egypt’s richest food-growing area. But when the Aswan dams were constructed in the 20th century to provide electricity and irrigation, and to protect the huge population centre of Cairo and its surrounding areas from annual flooding and drought, most of the sediment with its natural fertilizer accumulated up above the dam in the southern, upstream half of Lake Nasser, instead of passing down to the delta.
C Now, however, there turns out to be more to the story. It appears that the sediment-free water emerging from the Aswan dams picks up silt and sand as it erodes the river bed and banks on the 800-kilometre trip lo Cairo. Daniel Jean Stanley of the Smithsonian Institute noticed that water samples taken in Cairo, just before the river enters the delta, indicated that the river sometimes carries more than 850 grams of sediment per cubic metre of water - almost half of what it carried before the dams were built. ‘I’m ashamed to say that the significance of this didn't strike me until after I had read 50 or 60 studies,’ says Stanley in Marine Geology. "There is still a lot of sediment coming into the delta, but virtually no sediment comes out into the Mediterranean to replenish the coastline.So this sediment must be trapped on the delta itself.’
15D Once north of Cairo, most of the Nile water is diverted into more than 10,000 kilometres ofirrigation canals and only a small proportion reaches the sea directly through the rivers in the delta.The water in the irrigation canals is still or very slow-moving and thus cannot carry sediment, Stanley explains. The sediment sinks to the bottom of the canals and then is added to fields by farmers or pumped with the water into the four large freshwater lagoons that ore located near the outer edges of the delta. So very little of it actually reaches the coastline to replace what is being washed away by the Mediterranean currents.
16E The farms on the delta plains and fishing and aquaculture in the lagoons account for much of Egypt's food supply. But by the time the sediment has come to rest in the fields and lagoons it is laden with municipal, industrial and agricultural waste from the Cairo region, which is home to more than 40 million people. Pollutants are building up faster and faster,’ says Stanley. Based on his investigations of sediment from the delta lagoons, Frederic Siegel of George Washington University concurs.’ In Manzalah Lagoon, for example, the increase in mercury, lead and zinc coincided with the building of the High Dam at Aswan, the availability of cheap electricity, and the development of major power-based industries,’ he says. Since that time the concentration of mercury has increased significantly. Lead from engines that use leaded fuels and from other industrial sources has also increased dramatically. These poisons can easily enter the food chain, affecting the productivity of fishing and farming. Another problem is that agricultural wastes include fertilizers which stimulateincreases in plant growth in the lagoons and upset the ecology of the area, with serious effects on the fishing industry.
17F According to Siegel, international environmental organisations are beginning to pay closer attention to the region, partly because of the problems of erosion and pollution of the Nile delta, butprincipally because they fear the impact this situation could have on the whole Mediterraneancoastal ecosystem. But there are no easy solutions. In the immediate future, Stanley believes that one solution would be to make artificial floods to flush out the delta waterways, in the same way that natural floods did before the construction of the dams. He says, however, that in the long term an alternative process such as desalination may have to be used to increase the amount of water available. ‘In my view, Egypt must devise a way to have more water running through the river and thedelta,’ says Stanley. Easier said than done in a desert region with a rapidly growing population.
Step 1. 浏览所有题型，划分考察范围
关注大标题和图示，Disappearing Delta，确定主题词Nile delta等类似词汇会高频出现，不作为定位词。
Step 2. 先读正序题，后读乱序题
因为有了如上考察模式，我们可以先读正序题A1，划出定位词Egypt’s Mediterranean和考点词erosion occurred before Aswan dams，由于A段不出小标题，我们可以直接搜索判断题的细节定位词，定到A段前两句，发现考点吻合(in the past, land scoured away from…)，A1的答案为YES。
Step 3. 逐段阅读每段，完成每段会考察的题型，交叉做题(先正后乱)
现在到B段，很可能出判断题A2细节题，并且一定会出小标题B段主旨。于是先读正序题A2，划出定位词(Aswan dams)和考点词(people predicted… before dams were built)。回到B段，先读第一句话，发现后文是对首句话的细节补充，得出第一句话为主题句，标记TS(Topic Sentence)。然后顺着往后读，发现没有提到修建大坝之前，人们是否预测有海水侵蚀的事情。得出A2判断题答案NOT GIVEN。
B段后半截出现But也需要关注，但其实是通过介绍修大坝的目的，来呼应首句话，强调修大坝造成了泥沙被拦截，泥沙无法像之前那样自然地流到三角洲去填补侵蚀。于是回到前面划出的每个小标题关键词，得出B段主旨的小标题为iv. Interrupting a natural process。
因为完成了A2，现在回到判断题A3，划出定位词(Aswan dams)和考点词(考修建目的是不是increase fertility)。由于刚才读B段But后就发现了修大坝目的，是provide electricity and irrigation…，考点不吻合，答案是NO。