Exercises (b)

1. Establish compliance between parts of sentences:

1. for any drainage basin 1. if a stream order is plotted

a hierarchy of stream orders against the total stream lengths

2. streams without tributaries per order

at the head of river systems 2. are regular and

3.the examination of a large number of determinable

systems has shown that if the number 3. are designated first-order

of streams is counted within each order streams

4. similar straight-line plots 4. can be applied

can be obtained 5. then that number decreases

5. the bifurcation ratio is an expression with increasing order in regular

relating the number of streams manner

6. these various geometric properties 6. in a given order to the number

show that in many river basins in the next order

downstream changes

Text (b)

Read the text «Speciality of drainage basin» using a dictionary:

A drainage basin is an extent or an area of land where surface water from rain, melting snow, or iceconverges to a single point at a lower elevation, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean. For example, a tributary stream of a brook that joins a small river is tributary of a larger river, which is thus part of a series of successively smaller area but higher elevation drainage basins. Similarly, the Missouri and American rivers are each part of their own drainage basins and that of the Mississippi River.

Other terms that are used to describe drainage basins are catchment, catchment area, catchment basin, drainage area, river basin, and water basin. In North America, the term watershed is commonly used to mean a drainage basin, though in other English-speaking countries, it is used only in its original sense, to mean a drainage divide, the one meaning an area, the other its high elevation perimeter of that area. Drainage basins drain into other drainage basins in a hierarchical pattern, with smaller sub-drainage basinscombining into larger drainage basins.

In closed ("endorheic") drainage basins the water converges to a single point inside the basin, known as asink, which may be a permanent lake, dry lake, or a point where surface water is lost underground. The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels, and is separated from adjacent basins by a drainage divide.

The drainage basin acts as a funnel by collecting all the water within the area covered by the basin and channelling it to a single point. Each drainage basin is separated topographically from adjacent basins by a perimeter, the drainage divide making up a succession of higher geographical features (such as aridge, hill or mountains) forming a barrier.

Drainage basins are similar but not identical to hydrologic units, which are drainage areas delineated so as to nest into a multi-level hierarchical drainage system. Hydrologic units are designed to allow multiple inlets, outlets, or sinks. In a strict sense, all drainage basins are hydrologic units but not all hydrologic units are drainage basins.