A large volume of sediments has entered the coastal areas of the Ryukyu (Nansei) Islands, particularly Okinawa Island, as a result of large-scale agricultural land preparation and development projects. This has resulted in damage to the valuable ecosystem of corrals and seaweed beds and to the coastal landscape and fisheries. A water-quality survey was conducted over a four-year period at the Miyara River to measure suspended solids, nitrogen and other elements, and the river flow rate, so as to provide important information for planning load reduction measures and management methodologies for the catchment area. The volume of red soil and the total nitrogen loads transported by the rivers were determined, taking into account the frequent soil runoff and increases in nitrogen emissions associated with rainfall. The volume of fertilizer input was also estimated from existing data.