Econstudentlog

Rivers (I)

I gave the book one star on goodreads. My review on goodreads explains why. In this post I’ll disregard the weak parts of the book and only cover ‘the good stuff’. Part of the reason why I gave the book one star instead of two was that I wanted to punish the author for wasting my time with irrelevant stuff when it was clear to me that he could actually have been providing useful information instead; some parts of the book are quite good.

Some quotes and links below.

“[W]ater is continuously on the move, being recycled between the land, oceans, and atmosphere: an eternal succession known as the hydrological cycle. Rivers play a key role in the hydrological cycle, draining water from the land and moving it ultimately to the sea. Any rain or melted snow that doesn’t evaporate or seep into the earth flows downhill over the land surface under the influence of gravity. This flow is channelled by small irregularities in the topography into rivulets that merge to become gullies that feed into larger channels. The flow of rivers is augmented with water flowing through the soil and from underground stores, but a river is more than simply water flowing to the sea. A river also carries rocks and other sediments, dissolved minerals, plants, and animals, both dead and alive. In doing so, rivers transport large amounts of material and provide habitats for a great variety of wildlife. They carve valleys and deposit plains, being largely responsible for shaping the Earth’s continental landscapes. Rivers change progressively over their course from headwaters to mouth, from steep streams that are narrow and turbulent to wider, deeper, often meandering channels. From upstream to downstream, a continuum of change occurs: the volume of water flowing usually increases and coarse sediments grade into finer material. In its upper reaches, a river erodes its bed and banks, but this removal of earth, pebbles, and sometimes boulders gives way to the deposition of material in lower reaches. In tune with these variations in the physical characteristics of the river, changes can also be seen in the types of creatures and plants that make the river their home. […] Rivers interact with the sediments beneath the channel and with the air above. The water flowing in many rivers comes both directly from the air as rainfall – or another form of precipitation – and also from groundwater sources held in rocks and gravels beneath, both being flows of water through the hydrological cycle.”

“One interesting aspect of rivers is that they seem to be organized hierarchically. When viewed from an aircraft or on a map, rivers form distinct networks like the branches of a tree. Small tributary channels join together to form larger channels which in turn merge to form still larger rivers. This progressive increase in river size is often described using a numerical ordering scheme in which the smallest stream is called first order, the union of two first-order channels produces a second-order river, the union of two second-order channels produces a third-order river, and so on. Stream order only increases when two channels of the same rank merge. Very large rivers, such as the Nile and Mississippi, are tenth-order rivers; the Amazon twelfth order. Each river drains an area of land that is proportional to its size. This area is known by several different terms: drainage basin, river basin, or catchment (‘watershed’ is also used in American English, but this word means the drainage divide between two adjacent basins in British English). In the same way that a river network is made up of a hierarchy of low-order rivers nested within higher-order rivers, their drainage basins also fit together to form a nested hierarchy. In other words, smaller units are repeating elements nested within larger units. All of these units are linked by flows of water, sediment, and energy. Recognizing rivers as being made up of a series of units that are arranged hierarchically provides a potent framework in which to study the patterns and processes associated with rivers. […] processes operating at the upper levels of the hierarchy exert considerable influence over features lower down in the hierarchy, but not the other way around. […] Generally, the larger the spatial scale, the slower the processes and rates of change.”

The stuff above incidentally – and curiously – links very closely with the material covered in Holland’s book on complexity, which I finished just the day before I started reading this one. That book has a lot more stuff about things like nested hierarchies and that ‘potent framework’ mentioned above, and how to go about analyzing such things. (I found that book hard to blog – at least at first, which is why I’m right now covering this book instead; but I do hope to get to it later, it was quite interesting).

“Measuring the length of a river is more complicated than it sounds. […] Disagreements about the true source of many rivers have been a continuous feature of [the] history of exploration. […] most rivers typically have many tributaries and hence numerous sources. […] But it gets more confusing. Some rivers do not have a mouth. […] Some rivers have more than one channel. […] Yet another important part of measuring the length of a river is the scale at which it is measured. Fundamentally, the length of a river varies with the map scale because different amounts of detail are generalized at different scales.”

“Two particularly important properties of river flow are velocity and discharge – the volume of water moving past a point over some interval of time […]. A continuous record of discharge plotted against time is called a hydrograph which, depending on the time frame chosen, may give a detailed depiction of a flood event over a few days, or the discharge pattern over a year or more. […] River flow is dependent upon many different factors, including the area and shape of the drainage basin. If all else is equal, larger basins experience larger flows. A river draining a circular basin tends to have a peak in flow because water from all its tributaries arrives at more or less the same time as compared to a river draining a long, narrow basin in which water arrives from tributaries in a more staggered manner. The surface conditions in a basin are also important. Vegetation, for example, intercepts rainfall and hence slows down its movement into rivers. Climate is a particularly significant determinant of river flow. […] All the rivers with the greatest flows are almost entirely located in the humid tropics, where rainfall is abundant throughout the year. […] Rivers in the humid tropics experience relatively constant flows throughout the year, but perennial rivers in more seasonal climates exhibit marked seasonality in flow. […] Some rivers are large enough to flow through more than one climate region. Some desert rivers, for instance, are perennial because they receive most of their flow from high rainfall areas outside the desert. These are known as ‘exotic’ rivers. The Nile is an example […]. These rivers lose large amounts of water – by evaporation and infiltration into soils – while flowing through the desert, but their volumes are such that they maintain their continuity and reach the sea. By contrast, many exotic desert rivers do not flow into the sea but deliver their water to interior basins.”

…and in rare cases, so much water is contributed to the interior basin that that basin’s actually categorized as a ‘sea’. However humans tend to mess such things up. Amu Darya and Syr Darya used to flow into the Aral Sea, until Soviet planners decided they shouldn’t do that anymore. Goodbye Aral Sea – hello Aralkum Desert!

“An important measure of the way a river system moulds its landscape is the ‘drainage density’. This is the sum of the channel length divided by the total area drained, which reflects the spacing of channels. Hence, drainage density expresses the degree to which a river dissects the landscape, effectively controlling the texture of relief. Numerous studies have shown that drainage density has a great range in different regions, depending on conditions of climate, vegetation, and geology particularly. […] Rivers shape the Earth’s continental landscapes in three main ways: by the erosion, transport, and deposition of sediments. These three processes have been used to recognize a simple three-part classification of individual rivers and river networks according to the dominant process in each of three areas: source, transfer, and depositional zones. The first zone consists of the river’s upper reaches, the area from which most of the water and sediment are derived. This is where most of the river’s erosion occurs, and this eroded material is transported through the second zone to be deposited in the third zone. These three zones are idealized because some sediment is eroded, stored, and transported in each of them, but within each zone one process is dominant.”

“The flow of water carries […] sediment in three ways: dissolved material […] moves in solution; small particles are carried in suspension; and larger particles are transported along the stream bed by rolling, sliding, or a bouncing movement known as ‘saltation’. […] Globally, it is estimated that rivers transport around 15 billion tonnes of suspended material annually to the oceans, plus about another 4 billion tonnes of dissolved material. In its upper reaches, a river might flow across bedrock but further downstream this is much less likely. Alluvial rivers are flanked by a floodplain, the channel cut into material that the river itself has transported and deposited. The floodplain is a relatively flat area which is periodically inundated during periods of high flow […] When water spills out onto the floodplain, the velocity of flow decreases and sediment begins to settle, causing fresh deposits of alluvium on the floodplain. Certain patterns of alluvial river channels have been seen on every continent and are divided at the most basic level into straight, meandering, and braided. Straight channels are rare in nature […] The most common river channel pattern is a series of bends known as meanders […]. Meanders develop because erosion becomes concentrated on the outside of a bend and deposition on the inside. As these linked processes continue, the meander bend can become more emphasized, and a particularly sinuous meander may eventually be cut off at its narrow neck, leaving an oxbow lake as evidence of its former course. Alluvial meanders migrate, both down and across their floodplain […]. This lateral migration is an important process in the formation of floodplains. Braided rivers can be recognized by their numerous flows that split off and rejoin each other to give a braided appearance. These multiple intersecting flows are separated by small and often temporary islands of alluvium. Braided rivers typically carry abundant sediment and are found in areas with a fairly steep gradient, often near mountainous regions.”

“The meander cut-off creating an oxbow lake is one way in which a channel makes an abrupt change of course, a characteristic of some alluvial rivers that is generally referred to as ‘avulsion’. It is a natural process by which flow diverts out of an established channel into a new permanent course on the adjacent floodplain, a change in course that can present a major threat to human activities. Rapid, frequent, and often significant avulsions have typified many rivers on the Indo-Gangetic plains of South Asia. In India, the Kosi River has migrated about 100 kilometres westward in the last 200 years […] Why a river suddenly avulses is not understood completely, but earthquakes play a part on the Indo-Gangetic plains. […] Most rivers eventually flow into the sea or a lake, where they deposit sediment which builds up into a landform known as a delta. The name comes from the Greek letter delta, Δ, shaped like a triangle or fan, one of the classic shapes a delta can take. […] Material laid down at the end of a river can continue underwater far beyond the delta as a deep-sea fan.”

“The organisms found in fluvial ecosystems are commonly classified according to the methods they use to gather food and feed. ‘Shredders’ are organisms that consume small sections of leaves; ‘grazers’ and ‘scrapers’ consume algae from the surfaces of objects such as stones and large plants; ‘collectors’ feed on fine organic matter produced by the breakdown of other once-living things; and ‘predators’ eat other living creatures. The relative importance of these groups of creatures typically changes as one moves from the headwaters of a river to stretches further downstream […] small headwater streams are often shaded by overhanging vegetation which limits sunlight and photosynthesis but contributes organic matter by leaf fall. Shredders and collectors typically dominate in these stretches, but further downstream, where the river is wider and thus receives more sunlight and less leaf fall, the situation is quite different. […] There’s no doubting the numerous fundamental ways in which a river’s biology is dependent upon its physical setting, particularly in terms of climate, geology, and topography. Nevertheless, these relationships also work in reverse. The biological components of rivers also act to shape the physical environment, particularly at more local scales. Beavers provide a good illustration of the ways in which the physical structure of rivers can be changed profoundly by large mammals. […] rivers can act both as corridors for species dispersal but also as barriers to the dispersal of organisms.”

 

Drainage system (geomorphology).
Perennial stream.
Nilometer.
Mekong.
Riverscape.
Oxbow lake.
Channel River.
Long profile of a river.
Bengal fan.
River continuum concept.
Flood pulse concept.
Riparian zone.

 

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January 11, 2018 - Posted by | Books, Ecology, Geography, Geology

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