1. Establish compliance between parts of sentences:

1. the term "river" generally refers to 1. flbod magnitude and

2. rivers may carry water all the time data which is vital to the water

and are then called engineer

3. a stream or river is a body 2. on the geological and

of water flowing in a defined channel morphological сharacteristics

4. channel flow can be recorded of the catchment area.

in the form of a hydrograph, which 3. as distinct from surface run-off

5. from hydrographs a great deal on slopes

of information can be gained about 4. the large, main trunks of a

6. the variation in hydrograph shape drainage system

from river to river shows the 5. shows the variation of discharge

dependence of the discharge with time

6. perennial streams or rivers

Text (b)

Read the text «Water flows», using a dictionary:

When water flows in a stream it is subject to two basic forces. Gravity exerts an impelling force and is opposed by the frictional resistance between the water and the bed of the channel. A stream’s ability to work, that is, to erode and transport material, is related to these two forces. Potential energy is provided by the weight and elevation of the water. This is converted by gravity into downflow and hence into kinetic energy. However, something like 95 % of this energy is lost because of the frictional forces, and the precise shape and nature of the channel bed can have a significant effect on this figure.

In all but the most sluggish streams, water flow is not steady and uniform but is affected by turbulence, which takes the. Form of chaotic movements and eddies. Turbulence is an important flow characteristic because it creates upward water motion, which lifts and supports the finer sediments. The effect of friction ensiirei that water closest to the banks normally moves more slowly than that near the stream centre. The highest velocity is usually located in mid-stream about one third of the distance down from the surface to the bed, but in an asymmetrical channel the zone of maximum velocity shifts to the deeper side, and may cause - significant erosion. The discharge (velocity X cross-sectional area) of a stream is affected by the characteristics of the channel itself, particularly width, depth, and channel roughness.

The effeciency of the channel shape can be expressed by its hydraulic radius, a quantity defined as the ratio of the cross-sectional area (a) to the length of the wetted perimeter (p). The higher the ration the more efficient the stream and the smaller the loss due to the friction. Channel roughness can also have a marked effect. A rough channel creates considerable eddying and loss of energy, whereas a smooth channel minimizes the frictional loss. Channels in silt and clay tend to be deeper and narrower than those in sand and gravel, because the finer materials are cohesive and promote bank stability.