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Newsletter and Technical Publications
Lakes and Reservoirs vol. 2
The Watershed: Water from the
Mountains into the Sea
Streams And Rivers: Water Flowing Over the Land Surface
Rivers, cascades and streams are major hydrologic features of the global
landscape (Photo 20). They are the primary means by which precipitation and
snowmelt flow over the land surface to the oceans to begin the hydrologic cycle
anew. Water flow or drainage over the land surface also represents the part of
thehydrologic cycle of most concern to humans because it represents the primary
interface between water resources in nature and human use of this resource. In
fact, no other natural resource has been more tightly tied to the degree of
human settlement in different regions of the world.
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| Photo 20: Yatsubuchi cascade in the Kamo River, Japan |
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| Photo 21: Water saturated soil. |
The primary factor determining whether precipitation will seep underground
into the soil, or flow over it to streams and rivers, is the type of soil and
the extent to which it is already saturated with moisture. One way to envisage
this phenomenon is to consider a dry land surface onto which precipitation
begins falling. The dry soil will absorb the initial precipitation, thereby
“wetting ” the soil. If the precipitation volume and duration is sufficiently
large, the spaces between the particles comprising the soil will become filled
with water, thereby “saturating ” the soil (photo 21). Once the soil becomes
saturated, any additional precipitation will begin flowing over the land
surface, rather than seeping into it, eventually entering rivers, lakes,
wetlands, etc. Thus, the flow regime of rivers is directly dependent on the
precipitation patterns in their watersheds. In semi-arid and arid regions,
rivers like wadies
(Photos 22 and 23), may only flow intermittently. Low flows in humid areas may
be considerable, presenting frequent flood events. River flows can vary
significantly between these extremes, in both volume and timing. |
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Photos 22 and 23: Wadi in the wet and dry season, Syria,.
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Precipitation that does not seep into the ground will flow downhill,
eventually draining into streams that increase in size downstream. The streams
can eventually become large rivers transporting large volumes of water (as well
as dissolved and particulate material collected in its passage over the land
surface). The drainage channels (streams, rivers, etc.) typically occupy the
lowest part of the landscape. Thus, both the depth of the drainage channels and
the volume of water carried in them generally increases downstream in a
watershed. Other factors being equal, the volume of water carried in drainage
channels usually increases with increasing size and development of the
contributing watershed area.
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Fig. 3: Dendritic Shape of a river in a watershed.
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Because most rivers are part of a branched or
dendritic
drainage pattern, small creeks join to form larger streams downstream, much in
the same manner represented by branches in a tree or blood vessels in the human
body (Fig. 3 and Photo 24). Because flowing water can dissolve minerals and
otherwise erode water channels in the land surface, river flows have produced
dramatic modifications of the land surface over time. In fact, viewed from above
the land surface, rivers are never straight. Rather, because of the turbulent
nature of flowing water and the variability of land surface geology, many rivers
tend to form meandering channels. A prominent example is the Grand Canyon in the
western United States. This was carved out of the natural landscape over
geologic time as a result of erosion by the meandering Colorado River (Photo
25).
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| Photo 24: Mountain streams; originators of rivers. |
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| Photo 25: Colorado River and Grand Canyon, USA. |
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Figure 4 summarizes data on the availability of river water resources on the
various continents. It is noted that six countries (Brazil, Russia, Canada,
United States, China and India) contribute nearly half of the world’s total
river runoff to the oceans. The annual discharge of selected rivers from
different continents, comprising about 40% of the total global river runoff
volume, is given in Table 1.
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Fig.4: Availability of river water resources on the various continents
(figures in cubic kilometres per year).
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Table 1: Runoff of major rivers in the world, watershed population and
watershed land area.
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The volume of river water discharges generally reflects both the watershed
size and the prevailing patterns of the discharge as well as the land surface
patterns. Panama and Surinam, for example, have the world’s largest water
availability per square kilometre of land surface (1,870,000 and 1,411,000 cubic
kilometres of water/year, respectively). In contrast, Mauritania and Libya
exhibit the minimum water availability per square kilometre (390 and 3,010 cubic
kilometres of water/year, respectively). Per capita water availability also
varies widely around the world, being a function of the quantity of river flow
and the number of persons using the water.
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| Photo 26: River impoundment to control flooding. |
Most of the world’s major rivers are now impounded at some points in their
watersheds, primarily to counter the vagaries of their flows (Photo 26), and/or
to utilize their water to maximum human benefit although some environmental
impacts may occur (Photo 27). In some countries, virtually every feasible dam
site has been exploited, and the only remaining large, free-flowing rivers are
found in the North American and Russian tundra regions and in parts of Africa
and South America. River modifications have changed the natural flows of some
rivers to the extent that they no longer flow to the oceans during their dry
seasons (Photo 28). Prominent examples include the Yellow, Indus, Ganges, Nile,
and Colorado. In some cases, a watershed can become a closed or terminal water
system, in which no water flows from it to the oceans. |
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| Photo 27: Downstream turbulence after the water passing through the
turbines causing environmental impacts. Yaseita Dam, Argentina and Paraguay. |
Photo 28: River bed during the dry season, Brazil. |
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