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United Nations Environment Programme
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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.

Photo 20: Yatsubuchi cascade in the Kamo River, Japan


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.


Photos 22 and 23: Wadi in the wet and dry season, Syria,.

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.

Fig. 3: Dendritic Shape of a river in a watershed.

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).

Photo 24: Mountain streams; originators of rivers.
Photo 25: Colorado River and Grand Canyon, USA.

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.

Fig.4: Availability of river water resources on the various continents (figures in cubic kilometres per year).
Table 1: Runoff of major rivers in the world, watershed population and watershed land area. detail

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.

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.

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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