Econstudentlog

Rivers (II)

Some more observations from the book and related links below.

“By almost every measure, the Amazon is the greatest of all the large rivers. Encompassing more than 7 million square kilometres, its drainage basin is the largest in the world and makes up 5% of the global land surface. The river accounts for nearly one-fifth of all the river water discharged into the oceans. The flow is so great that water from the Amazon can still be identified 125 miles out in the Atlantic […] The Amazon has some 1,100 tributaries, and 7 of these are more than 1,600 kilometres long. […] In the lowlands, most Amazonian rivers have extensive floodplains studded with thousands of shallow lakes. Up to one-quarter of the entire Amazon Basin is periodically flooded, and these lakes become progressively connected with each other as the water level rise.”

“To hydrologists, the term ‘flood’ refers to a river’s annual peak discharge period, whether the water inundates the surrounding landscape or not. In more common parlance, however, a flood is synonymous with the river overflowing it’s banks […] Rivers flood in the natural course of events. This often occurs on the floodplain, as the name implies, but flooding can affect almost all of the length of the river. Extreme weather, particularly heavy or protracted rainfall, is the most frequent cause of flooding. The melting of snow and ice is another common cause. […] River floods are one of the most common natural hazards affecting human society, frequently causing social disruption, material damage, and loss of life. […] Most floods have a seasonal element in their occurence […] It is a general rule that the magnitude of a flood is inversely related to its frequency […] Many of the less predictable causes of flooding occur after a valley has been blocked by a natural dam as a result of a landslide, glacier, or lava flow. Natural dams may cause upstream flooding as the blocked river forms a lake and downstream flooding as a result of failure of the dam.”

“The Tigris-Euphrates, Nile, and Indus are all large, exotic river systems, but in other respects they are quite different. The Nile has a relatively gentle gradient in Egypt and a channel that has experienced only small changes over the last few thousand years, by meander cut-off and a minor shift eastwards. The river usually flooded in a regular and predictable way. The stability and long continuity of the Egyptian civilization may be a reflection of its river’s relative stability. The steeper channel of the Indus, by contrast, has experienced major avulsions over great distances on the lower Indus Plain and some very large floods caused by the failure of glacier ice dams in the Himalayan mountains. Likely explanations for the abandonment of many Harappan cities […] take account of damage caused by major floods and/or the disruption caused by channel avulsion leading to a loss of water supply. Channel avulsion was also a problem for the Sumerian civilization on the alluvial plain called Mesopotamia […] known for the rise and fall of its numerous city states. Most of these cities were situated along the Euphrates River, probably because it was more easily controlled for irrigation purposes than the Tigris, which flowed faster and carried much more water. However, the Euphrates was an anastomosing river with multiple channels that diverge and rejoin. Over time, individual branch channels ceased to flow as others formed, and settlements located on these channels inevitably declined and were abandoned as their water supply ran dry, while others expanded as their channels carried greater amounts of water.”

“During the colonization of the Americas in the mid-18th century and the imperial expansion into Africa and Asia in the late 19th century, rivers were commonly used as boundaries because they were the first, and frequently the only, features mapped by European explorers. The diplomats in Europe who negotiated the allocation of colonial territories claimed by rival powers knew little of the places they were carving up. Often, their limited knowledge was based solely on maps that showed few details, rivers being the only distinct physical features marked. Today, many international river boundaries remain as legacies of those historical decisions based on poor geographical knowledge because states have been reluctant to alter their territorial boundaries from original delimitation agreements. […] no less than three-quarters of the world’s international boundaries follow rivers for at least part of their course. […] approximately 60% of the world’s fresh water is drawn from rivers shared by more than one country.”

“The sediments carried in rivers, laid down over many years, represent a record of the changes that have occurred in the drainage basin through the ages. Analysis of these sediments is one way in which physical geographers can interpret the historical development of landscapes. They can study the physical and chemical characteristics of the sediments itself and/or the biological remains they contain, such as pollen or spores. […] The simple rate at which material is deposited by a river can be a good reflection of how conditions have changed in the drainage basin. […] Pollen from surrounding plants is often found in abundance in fluvial sediments, and the analysis of pollen can yield a great deal of information about past conditions in an area. […] Very long sediment cores taken from lakes and swamps enable us to reconstruct changes in vegetation over very long time periods, in some cases over a million years […] Because climate is a strong determinant of vegetation, pollen analysis has also proved to be an important method for tracing changes in past climates.”

“The energy in flowing and falling water has been harnessed to perform work by turning water-wheels for more than 2,000 years. The moving water turns a large wheel and a shaft connected to the wheel axle transmits the power from the water through a system of gears and cogs to work machinery, such as a millstone to grind corn. […] The early medieval watermill was able to do the work of between 30 and 60 people, and by the end of the 10th century in Europe, waterwheels were commonly used in a wide range of industries, including powering forge hammers, oil and silk mills, sugar-cane crushers, ore-crushing mills, breaking up bark in tanning mills, pounding leather, and grinding stones. Nonetheless, most were still used for grinding grains for preparation into various types of food and drink. The Domesday Book, a survey prepared in England in AD 1086, lists 6,082 watermills, although this is probably a conservative estimate because many mills were not recorded in the far north of the country. By 1300, this number had risen to exceed 10,000. [..] Medieval watermills typically powered their wheels by using a dam or weir to concentrate the falling water and pond a reserve supply. These modifications to rivers became increasingly common all over Europe, and by the end of the Middle Ages, in the mid-15th century, watermills were in use on a huge number of rivers and streams. The importance of water power continued into the Industrial Revolution […]. The early textile factories were built to produce cloth using machines driven by waterwheels, so they were often called mills. […] [Today,] about one-third of all countries rely on hydropower for more than half their electricity. Globally, hydropower provides about 20% of the world’s total electricity supply.”

“Deliberate manipulation of river channels through engineering works, including dam construction, diversion, channelization, and culverting, […] has a long history. […] In Europe today, almost 80% of the total discharge of the continent’s major rivers is affected by measures designed to regulate flow, whether for drinking water supply, hydroelectric power generation, flood control, or any other reason. The proportion in individual countries is higher still. About 90% of rivers in the UK are regulated as a result of these activities, while in the Netherlands this percentage is close to 100. By contrast, some of the largest rivers on other continents, including the Amazon and the Congo, are hardly manipulated at all. […] Direct and intentional modifications to rivers are complemented by the impacts of land use and land use changes which frequently result in the alteration of rivers as an unintended side effect. Deforestation, afforestation, land drainage, agriculture, and the use of fire have all had significant impacts, with perhaps the most extreme effects produced by construction activity and urbanization. […] The major methods employed in river regulation are the construction of large dams […], the building of run-of-river impoundments such as weirs and locks, and by channelization, a term that covers a range of river engineering works including widening, deepening, straightening, and the stabilization of banks. […] Many aspects of a dynamic river channel and its associated ecosystems are mutually adjusting, so a human activity in a landscape that affects the supply of water or sediment is likely to set off a complex cascade of other alterations.”

“The methods of storage (in reservoirs) and distribution (by canal) have not changed fundamentally since the earliest river irrigation schemes, with the exception of some contemporary projects’ use of pumps to distribute water over greater distances. Nevertheless, many irrigation canals still harness the force of gravity. Half the world’s large dams (defined as being 15 metres or higher) were built exclusively or primarily for irrigation, and about one-third of the world’s irrigated cropland relies on reservoir water. In several countries, including such populous nations as India and China, more than 50% of arable land is irrigated by river water supplied from dams. […] Sadly, many irrigation schemes are not well managed and a number of environmental problems are frequently experienced as a result, both on-site and off-site. In many large networks of irrigation canals, less than half of the water diverted from a river or reservoir actually benefits crops. A lot of water seeps away through unlined canals or evaporates before reaching the fields. Some also runs off the fields or infiltrates through the soil, unused by plants, because farmers apply too much water or at the wrong time. Much of this water seeps back into nearby streams or joins underground aquifers, so can be used again, but the quality of water may deteriorate if it picks up salts, fertilizers, or pesticides. Excessive applications of irrigation water often result in rising water tables beneath fields, causing salinization and waterlogging. These processes reduce crop yields on irrigation schemes all over the world.”

“[Deforestation can contribute] to the degradation of aquatic habitats in numerous ways. The loss of trees along river banks can result in changes in the species found in the river because fewer trees means a decline in plant matter and insects falling from them, items eaten by some fish. Fewer trees on river banks also results in less shade. More sunlight reaching the river results in warmer water and the enhanced growth of algae. A change in species can occur as fish that feed on falling food are edged out by those able to feed on algae. Deforestation also typically results in more runoff and more soil erosion. This sediment may cover spawning grounds, leading to lower reproduction rates. […] Grazing and trampling by livestock reduces vegetation cover and causes the compaction of soil, which reduces its infiltration capacity. As rainwater passes over or through the soil in areas of intensive agriculture, it picks up residues from pesticides and fertilizers and transport them to rivers. In this way, agriculture has become a leading source of river pollution in certain parts of the world. Concentration of nitrates and phosphates, derived from fertilizers, have risen notably in many rivers in Europe and North America since the 1950s and have led to a range of […] problems encompassed under the term ‘eutrophication’ – the raising of biological productivity caused by nutrient enrichment. […] In slow-moving rivers […] the growth of algae reduces light penetration and depletes the oxygen in the water, sometimes causing fish kills.”

“One of the most profound ways in which people alter rivers is by damming them. Obstructing a river and controlling its flow in this way brings about a raft of changes. A dam traps sediments and nutrients, alters the river’s temperature and chemistry, and affects the processes of erosion and deposition by which the river sculpts the landscape. Dams create more uniform flow in rivers, usually by reducing peak flows and increasing minimum flows. Since the natural variation in flow is important for river ecosystems and their biodiversity, when dams even out flows the result is commonly fewer fish of fewer species. […] the past 50 years or so has seen a marked escalation in the rate and scale of construction of dams all over the world […]. At the beginning of the 21st century, there were about 800,000 dams worldwide […] In some large river systems, the capacity of dams is sufficient to hold more than the entire annual discharge of the river. […] Globally, the world’s major reservoirs are thought to control about 15% of the runoff from the land. The volume of water trapped worldwide in reservoirs of all sizes is no less than five times the total global annual river flow […] Downstream of a reservoir, the hydrological regime of a river is modified. Discharge, velocity, water quality, and thermal characteristics are all affected, leading to changes in the channel and its landscape, plants, and animals, both on the river itself and in deltas, estuaries, and offshore. By slowing the flow of river water, a dam acts as a trap for sediment and hence reduces loads in the river downstream. As a result, the flow downstream of the dam is highly erosive. A relative lack of silt arriving at a river’s delta can result in more coastal erosion and the intrusion of seawater that brings salt into delta ecosystems. […] The dam-barrier effect on migratory fish and their access to spawning grounds has been recognized in Europe since medieval times.”

“One of the most important effects cities have on rivers is the way in which urbanization affects flood runoff. Large areas of cities are typically impermeable, being covered by concrete, stone, tarmac, and bitumen. This tends to increase the amount of runoff produced in urban areas, an effect exacerbated by networks of storm drains and sewers. This water carries relatively little sediment (again, because soil surfaces have been covered by impermeable materials), so when it reaches a river channel it typically causes erosion and widening. Larger and more frequent floods are another outcome of the increase in runoff generated by urban areas. […] It […] seems very likely that efforts to manage the flood hazard on the Mississippi have contributed to an increased risk of damage from tropical storms on the Gulf of Mexico coast. The levées built along the river have contributed to the loss of coastal wetlands, starving them of sediment and fresh water, thereby reducing their dampening effect on storm surge levels. This probably enhanced the damage from Hurricane Katrina which struck the city of New Orleans in 2005.”

Links:

Onyx River.
Yangtze. Yangtze floods.
Missoula floods.
Murray River.
Ganges.
Thalweg.
Southeastern Anatolia Project.
Water conflict.
Hydropower.
Fulling mill.
Maritime transport.
Danube.
Lock (water navigation).
Hydrometry.
Yellow River.
Aswan High Dam. Warragamba Dam. Three Gorges Dam.
Onchocerciasis.
River restoration.

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January 16, 2018 - Posted by | Biology, Books, Ecology, Engineering, Geography, Geology, History

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