The watery, heavily vegetated marshlands of the Mekong Delta served as a formidable natural defense for Viet Cong units in the region, and the waterways served as the transportation corridor for Communist forces.. The Mobile Riverine Force formed the bulk of what came to be known as the Brown Water Navy, and was key to American efforts in the region. Elsewhere in Vietnam, dense jungle growth also presented barriers for rapid movement of allied troops, resulting in helicopters taking increasingly important roles with the military, serving as troop transports, cargo carriers, medical evacuation units, and even aerial gunships.