Starting in the central mountain range, the Kidron Valley wanders along the eastern edge of the city of Jerusalem, through the Judean dessert and eventually makes it way to the Dead Sea. Along its 32 kilometer (20 mile) course, it drops over 1200 meters (4000 feet). For the most part, the area in which the Kidron Valley flows does not receive any rain between the months of April and September. Consequently, the valley floor becomes very hard. In the fall, when the rains come, the water rushes down the valley. By the time it gets in the region of the Dead Sea, large amounts of water are rushing down and frequently cause flash flooding. Despite efforts of scientists and engineers to control the flow of water, these flash floods usually cause massive amounts of erosion and can even wash out roads.
In the Bible, the Kidron Valley is referenced a number of times as it ran between the city of Jerusalem and the Mount of Olives. In 2 Kings 23, as King Josiah was making reforms in the city of Jerusalem, he burned the articles of Baal and Asherah in the Kidron Valley. Later, water from the Gihon Spring once ran into the Kidron Valley until King Hezekiah diverted the water through a tunnel to the western side of the city (2 Kings 20:20, 2 Chronicles 32:4,30). During New Testament times, Jesus crossed the Kidron Valley a number of times as he made his way between Bethany and Jerusalem. In fact, Jesus would have crossed the Kidron Valley at least three times the day before he was crucified (once as He came into the city to partake of the Passover Feast, a second time when He went with the apostles to pray in the Garden of Gethsemane and finally as He was led away to his first trials before Annas and Caiaphas).
Today, the floor of the Kidron Valley in Jerusalem is around 10 meters higher than in Biblical times. This is due to all of the rubble and earth that has been pushed into the valley from the number of times that Jerusalem has been destroyed.
The top picture is a view looking south down the Kidron Valley from just north of the Garden of Gethsemane. The city of Jerusalem rises to the right and the Mount of Olives rise to the left. The bottom picture is from the area of the Pool of Siloam looking toward the northeast up the Kidron Valley. The Arab village of Silwan is on the right, with the Mount of Olives directly ahead. The city of Jerusalem is above the valley off of the left side of the picture.