The northwestern portion of Rajasthan is generally sandy and dry. Most of this region are covered by the Thar Desert which extends into adjoining portions of Pakistan. The Aravalli Range does not intercept the moisture-giving southwest monsoon winds off the Arabian Sea, as it lies in a direction parallel to that of the coming monsoon winds, leaving the northwestern region in a rain shadow. The Thar Desert is thinly populated; the town of Jodhpur is the largest city in the desert and known as the gateway of thar desert. The desert has some major districts like Jodhpur, Jaisalmer, Barmer, Bikaner and Nagour. This area is also important defence point of view. Jodhpur airbase is Indias largest airbase and military, BSF bases are also situated here. A single civil airport is also situated in Jodhpur. The Northwestern thorn scrub forests lie in a band around the Thar Desert, between the desert and the Aravallis. This region receives less than 400 mm of rain in an average year. Temperatures can exceed 48 °C in the summer months and drop below freezing in the winter. The Godwar, Marwar, and Shekhawati regions lie in the thorn scrub forest zone, along with the city of Jodhpur. The Luni River and its tributaries are the major river system of Godwar and Marwar regions, draining the western slopes of the Aravallis and emptying southwest into the great Rann of Kutch wetland in neighboring Gujarat. This river is saline in the lower reaches and remains potable only up to Balotara in Barmer district. The Ghaggar River, which originates in Haryana, is an intermittent stream that disappears into the sands of the Thar Desert in the northern corner of the state and is seen as a remnant of the primitive Saraswati river.