Nahal Sorek lies at the heart of one of the largest and most important nature reserves in the center of the country. For most of its known existence, it was a disappointing stream that revived only after the occasional wintertime flash flood. But for decades now, the sewage of Jerusalem has been flowing copiously through it.

In recent years, the sewage has undergone a purification process before it reaches the stream, but it still pollutes the water. And heavy rains actually make the situation worse, because the purification plant is unable to absorb the increased amount of sewage-tainted water.

The Israel Nature and Parks Authority, along with the state-run Water Authority, have recently launched a project aimed at removing the sewage from the stream and using it for crops in nearby agricultural areas in Beit Shemesh and Adulam. That would prevent the sewage from sinking into the ground as it runs through Nahal Sorek, which is surrounded by natural tree groves and a large variety of wild plants and animals.

"There was a time when we thought that after so many years of set periods of water flow, it was best to leave the situation alone, but we came to understand that there is no way to prevent instances of breakdowns when raw sewage flows through the stream and causes damage," says Yehoshua Shkedy, head of the parks authority's science division.

The Water Authority is examining proposals to carry out the project in cooperation with the ministries of the interior, environmental protection, health and agriculture - an achievement in itself, considering the many times these government offices have disagreed on projects involving water resources.

The idea is to have the water diverted to reservoirs near Beit Shemesh, where experts will examine the possibility of using the sometimes powerful stream as a source of hydroelectric energy. The water will undergo further purification and be used to water crops in the southern Adulam area.

"Some of this water, as well as natural water existing in the area, will be directed toward a site called the Tzora quarry," Shkedy says. "This is a place that was once a quarry, but which has filled with water and attracts birds. The Water Authority will help us channel water from Nahal Sorek to other spots as well."

The Sorek project is only one of several plans to divert water, which the parks and water authorities are compelled to do in an effort to help the country's springs and streams maintain a good water supply. In the Upper Galilee, for example, water is conveyed by a pipe from Nahal Dan to nearby Nahal Iyon, after the latter dried up.

Water also flows down Nahal David in the Judean Desert, but is then pumped back to allow farmers to use it. Nahal Kziv and the Ein Afek Nature Reserve in the Galilee receive water supplies from pipes supplied by the Mekorot water company. Water flows in the sources of these streams and rivers, but is pumped back up to the top to avoid waste when it flows downhill.