Bồng Sơn, ran northward along the coast into I Corps. Rarely more than 25 km wide, it consisted of a series of small deltas, which often backed into gently rolling terraces some 30-90m in height, and, at irregular intervals, of a number of mountainous spurs from the highlands. These spurs created narrow river valleys with steep ridges that frequently provided hideouts for PAVN/VC units or housed PAVN/VC command, control and logistical centers. The plain itself was bisected by the east-west Lai Giang River, which was in turn fed by two others, the An Lao, flowing from the northwest and the Kim Son, flowing from the southwest. These two rivers formed isolated but fertile valleys west of the coastal plain. The climate in the region was governed by the northeast monsoon. The heaviest rains had usually ended by December, but a light steady drizzle, which the French had called crachin weather and occasional torrential downpours could be expected to occur through March. These weather systems would at time limits the availability of air support.