The melting of the mountain snow-caps in the spring causes severe freshets, which in turn are followed by long seasons of drought at a time when water is most needed for agricultural purposes.
The Nogal sends down a turbulent stream during the freshets, while the Shebeli, notwithstanding the far greater extent of its basin, does not reach the sea.
It is deep, fairly rapid, subject to a regular rise and flood every autumn, but not to sudden freshets, and is affected by the tide 50 m.
Freshets in the spring of 1900, however, undermined the wall, and on the 7th of April the dam broke with a resulting loss of several lives and about $1,000,000 worth of property.
As the freshets begin to lessen and retire into the deeper channels, the currents form natural embankments on their edges, preventing the return of a small portion of water which is thus left stagnant on the sands, and exposed to the action of the sun's rays.
The Mareb is dry for a great part of the year, but like the Takazze is subject to sudden freshets during the rains.
The rivers themselves, of which there are as many as fifty, are little more than mountain torrents, all rising on the northern slopes of Elburz, flowing mostly in independent channels to the Caspian, and subject to sudden freshets and inundations along their lower course.
Freshets and droughts at times work havoc. The former made notable 1844 and 1858; and the latter 1860, 1874 and 1894.
Stream valleys and bottom lands are the conspicuous modifying feature of the prairie region; but in general, owing to the gentle slope of the streams and the great breadth of the plains, erosion has been slight; and indeed the streams, overloaded in seasonal freshets, are building up their valley floors.
In the spring freshets it is a magnificent stream, but in summer its volume greatly shrinks, and it is normally a broad, shallow, sluggish, stream, flowing through interlacing channels among the sand-bars it heaps athwart its course.