Unit 5. Drainage Basin Geometry Exercises (a)

1. Learn new words:

quantitative – количественный law – закон

investigation – исследование, изучение to obtain – получать

geometric properties – геометрические basin order – порядок

свойства речного бассейна

fluvial morphometry – речная bifurcation ratio – бифур-

морфометрия кационное отношение

drainage basin – водосборный бассейн drainage density – густота

to propose – предлагать речной сети

tributary – приток to decrease – уменьшать

first order stream – река первого ряда

head of river – исток реки

examination – исследование

1. Translate the following word combinations:

the relationships between the various components of river systems; to

facilitate comparisons between basins; in recent years; the quantitative investigation of the geometric properties of rivers; to be called fluvial morphometry; for any drainage basin; a hierarchy of stream orders; to be originally proposed by; the amended systems of Shreve or Strahler; streams without tributaries; at the head of river systems; two first order streams; to make a third order stream; to be named from the highest order stream in the basin; the examination of a large number of systems; to be counted within each order; with increasing order in a regular manner; the law of stream numbers; similar straight-line plots; against the total stream lengths; law of stream lengths; an expression relating the number of streams; law of basin areas; in the next order; various geometric properties; downstream changes; to be regular and determinable; from the analysis of drainage basins; to be found on resistant rocks in the Appalachians; a measure of the texture of the drainage net.

Text (a)

Read the text «Drainage Basin Geometry»:

In order to understand more fully the relationships between the various components of river systems, and to facilitate comparisons between basins, geomorphologists in recent years have placed considerable emphasis on the quantitative investigation of the geometric properties of rivers and their basins. This type of analysis is called fluvial morphometry. For any drainage basin a hierarchy of stream orders can be applied. Although the idea was originally proposed by R. E. Horton, the amended systems of Shreve or Strahler are the ones generally used. Streams without tributaries at the head of river systems are designated first-order streams. Two first order streams join to make a third order stream, and so on. If a lower order unites with a higher order stream, the order of the latter remains unchanged. The basin order is named from the highest order stream in the basin.

The examination of a large number of systems has shown that if the number of streams is counted within each order, then that number decreases with increasing order in a regular manner. This is known as the law of stream numbers (Horton, 1945). Similar straight-line plots can be obtained if a stream order is plotted against the total stream lengths, per order (law of stream lenghts) (Horton, 1945) and against the total area drained by each order (law of basin areas) (Schumm, 1956). The bifurcation ratio is an expression relating the number of streams in a given order to the number in the next order. These various geometric properties show that in many river basins downstream changes are regular and determinable. The laws described above are known collectively as the laws of drainage composition.

Other useful indices that can be derived from the analysis of drainage basins include an index of drainage density (Horton, 1932), expressed as the total channel length divided by the total area of the basin. In the United States, wide ranges of density value occur: the lowest 2 to 2.5 km per square kilometer are found on resistant rocks in the Appalachians; values in excess of 200 are reported from badlands. Stream frequency is the number of channels per unit area: like drainage density, it is a measure of the texture of the drainage net.