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.