雅思阅读第096套P3-Waterfalls

Waterfalls

Waterfalls are places where rivers or streams direct their flow over vertical drops. They have always been a lure for their scenic beauty or, in the case of the biggest, their ability to showcase nature’s might and majesty. Niagara Falls, on the border of Canada and America (discharging the most water of all), is a magnet for visitors, as is Victoria Falls, also straddling an international boundary between Zimbabwe and Zambia, and presenting the single largest sheet of falling water in the world. Similarly, the remoteness and inaccessibility of the highest waterfall, Angel Falls, located deep in the middle of the Venezuelan jungle, has not stopped it from becoming one of the country’s top tourist attractions.

There are many possible causes of waterfalls, but a common one is differences in rock type. When a river flows over a resistant rock bed, erosion is slow, but with the complex geological faulting of the Earth’s surface, softer patches of rock can be exposed. The water cuts into this, resulting in a minor turbulence at the boundary, stirring up pebbles and grit from the riverbed, which increases the erosive capacity of the current. And so a process begins whereby the river takes on two tiers, or levels, and a waterfall is born. Other more abrupt causes of waterfalls are earthquakes or landslides, which create fault lines in the land, or divert watercourses, respectively. Additionally, during past ice ages, glaciers scoured out many deep basins. These glaciers may have disappeared, but their feeder rivers can continue to flow as waterfalls into the remaining depressions.

Obviously then, waterfalls come in a variety of shapes and sizes, as different as the local geology in which they are found, and this has resulted in an abundance of descriptive terms. The word ‘cataract’ refers simply to a large powerful waterfall, while a ‘cascade’ descends a series of rock steps. If these steps are very distinct, it is a ‘tiered waterfall’, and if each step is larger still, of approximately the same size, and with a significant pool of water at each base, it is known as a multi-step waterfall’. If the falling water engages with the rock face, it often widens, to be called a ‘horsetail waterfall’, while if it does not touch the rock face at all, it is a ‘plunge waterfall’ — often the most picturesque.

Regardless of such differences, all waterfalls have in common a vertical height and average flow of water. These features, taken together, are a measure of the waterfall’s power, quantified using a ten-point logarithmic scale. Giant falls, such as Niagara, are graded at the very top of this scale, find smaller falls, which may occur in town creeks, at the bottom. Another common feature of larger falls is a ‘plunge pool’. This is caused by the rubble at the base of the falls, which is stirred and broken into smaller pieces. In the never-ending eddies and whirlpools, these pieces scour out a deep underwater basin. An interesting consequence is that such falls are in the process of retreat, since the softer material at the lower face suffers undercutting. This gives rise to rock shelters behind the falling water, which steadily become larger until the roof collapses, and the waterfall retreats significantly backward into the Earth.

Of course, to people at large, a waterfall seems fixed and forever. Erosion is indeed a slow process; however, given a sufficiently powerful waterfall and the right sort of rock, the retreat can be over a meter a year. This would be clearly observable over a person’s life time, and a fast-motion view, spanning several decades, would see an essentially unchanged height of falling water burrowing backwards with surprising evenness. Since this motion is towards higher elevations or through more hilly terrain, a host of geological features can be laid in the waterfall’s retreating path. Victoria Falls are a prime example, with its lower reaches characterised by spectacular islands, gorges, and rock formations.

This retreat occasionally causes problems, as can be seen with Niagara Falls. In just over ten millennia, the falls have moved almost 11 kilometres upstream. Since the Niagara river marks the border of Canada and America, as agreed in 1819, the detectable retreat of these falls since that time technically means that the Canadian frontier has advanced forward at the expense of America, although this argument has obviously caused dispute. More practically, with so much infrastructure, such as hotels, roads, bridges, and scenic viewpoints, all rigidly established, it remains important to limit the erosion. For this reason, the exposed ridges of the falls have been extensively strengthened, and underwater barriers installed to divert the more erosive of river currents.

The most ambitious erosion-control measure took place in 1969 on Niagara’s American Falls, whose retreat was nibbling away at American territory. The branch of the Niagara river which feeds these subsidiary falls was dammed, allowing the main Horseshoe Falls to absorb the excess flow. The then-completely-dry-and-exposed river bottom and cliff face allowed a team of US-army engineers to use bolts, cement, and brackets, to strengthen any unstable rock. Five months later, the temporary dam was destroyed with explosives, returning water to the falls, but with the inexorable erosion process having been slowed considerably.

SECTION 3: QUESTIONS 27-40

Questions 27-31

Write TRUE, FALSE, or NOT GIVEN.

TRUE	if the statement agrees with the information
FALSE	if the statement contradicts the information
NOT GIVEN	If there is no information on this