Hi,
This week rains made Beit Zait Dam overflow for the first time in three years 
. The dam was built on 1955 in order to collect the water flowing down Nahal Sorek (Sorek stream) and made it seep down into the Mountain Aquifer. Back then it was the biggest dam in Israel, to create the biggest lake in Israel.
The dam created an artificial lake behind it but checks made to the Mountain Aquifer showed it did not changed. So where did the water go to? Nahal Sorek is flowing down to the west into the Mediterranean Sea. But the Topographic drainage divide (above the ground) is different from the Geological drainage divide (under the ground).
Meaning that the water seeping down from the lake are flowing to the East, towards the Dead Sea. The water got out in the spring East of Jerusalem, in an area which was back then in Jordanian occupation. So instead of supporting the young state water reservoirs, it supported the water reservoir of one of the enemy countries.
It changed in Six Day War when Israel liberated the West Bank and owned the area as well as the water in its ground.
But it was decided not to demolish the dam even after the mistake was found. The place is a touristic attraction during the winter season, as it the only (altougth artificial) lake in the area. People run, hike, bike, picnic and sometimes swim or paddle in the lake.
Now days there are new calls to demolish it as hurt the ecosystem, mainly mosquitos at the spring time. There were also cases of drowning in the lake due the muddy banks.
Take Care
Gad
The dam and the lake behind it
The lake during hot winter days…
… becomes a great spot for family picnics
Overflow of the water over the dam (by Ilan Rosenberg)
The dam (back on 2010). Buildings in Israel are desinged to stand 50 years old, I do not know about the dam (as we do not have many of those here) – you can see the concrete is wearing as it a 70 years old structure.
David Ben Gurion planting a Cedar tree in Jerusalem forest on 1958 above Beit Zait Dam (the tree is now dying)
The Beit Zait dam and the like behind it on 1968