Fig. 9.4. Asymmetric river development.
The orientation of a river relative to the Sun's
position affects the nutrient supply. Where the
river flows east > west or west > east, the side
nearest the Sun tends to be more shaded and the
water cooler; a positive temperature gradient
develops, allowing the cooler ground to absorb
mineral-rich waterfront the river, and the soil
becomes more fertile. On the side exposed to the
Sun, the reverse occurs, with a negative
temperature gradient forcing groundwater, with its
minerals to leach into the river.trees die off, because the improved soil conditions don't suit them.
Other species of tree can replace them and the dynamic balance of
Nature is restored.
Irrigation in hot climates aggravates the problem because, as the
ground temperatures cool during the night, the irrigating water can
penetrate the upper salt-containing strata. With the increase in tem-
perature during the day, the infiltrated irrigation water with its
acquired salts are drawn up, and upon exposure to light and heat are
deposited on the soil surface. The seriousness of the problem will
vary with latitude, height and season.
All healthy rivers will carry nutrients in suspension that will
be absorbed by the vegetation on the river banks if the soil is
cooler than the river water. This improves soil fertility and
recharges the groundwater. But, if the soil is warmer than the
river, due to the absence of protective cover, a negative tempera-
ture gradient will cause the nutrients to leach from the soil into
the river, which will eventually make the soil sterile and unpro-
ductive. The longer a river flows through irrigated, sunlit farm-
lands, the more it becomes contaminated with salts, artificial124 HIDDEN NATURE