Course
The headwaters of the Zarqa begin just northeast of Amman, rising from a spring named Ain Ghazal ("Gazelle spring"). The river flows to the north before heading west. Rising on the eastern side of the mountains of Gilead, it runs a course of about 105 kilometers (65 mi) in a wild and deep ravine before flowing into the Jordan River between Gennesaret and the Dead Sea, at a point 1,090 meters (3,576 ft) below its origin. At its higher reaches, the river banks are mostly steep and canyon-like. Near Ain Ghazal, two tributary wadis join the river, and it opens up into a shallow basin. It forms the border between the Jordanian administrative regions of Irbid and Al Balqa.
The river is perennial, but with a very low base flow of about 2–million to 3 million cubic meters per month during the summer months, and as much as 5–million to 8 million cubic meters per month during the rainy winter months. This makes it the second largest tributary of the lower Jordan River, after the Yarmouk River, and the third largest river in the region by annual discharge. Irregular floods after rain storms may increase the flow to as much as 54 million cubic meters. The median annual flow is 63.3 million cubic meters. The total basin area is 3,900 km2 (1,500 sq mi) the largest in Jordan. A small dam, Al-Rwyha dam, near the village of Dayr Alla, marks the end of the upstream portion of the river, where it is natural and fast flowing with very clear water. There is very little agriculture along the banks of the river in this region, which are very rocky. Downstream from this dam, the water level is very low, and the river banks are intensively used for agriculture, as well as grazing by sheep and goats
The King Talal Dam was built across the lower Zarqa in 1970, and created a reservoir with a capacity of 55 million cubic meters, and increased in 1987 to 86 million cubic meters. When built, it was expected that the reservoir would supply water for municipal use in the Amman region. However, the current levels of pollution in the lake make the water unfit for human consumption, and it is used for irrigation only.
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