<P> The small amount of habitable land has prompted significant human modification of the terrain over many centuries . Land was reclaimed from the sea and from river deltas by building dikes and drainage, and rice paddies were built on terraces carved into mountainsides . The process continued in the modern period with extension of shorelines and building of artificial islands for industrial and port development, such as Port Island in Kobe and the new Kansai International Airport in Osaka Bay . Hills and even mountains have been razed to provide flat areas for housing . </P> <P> Rivers are generally steep and swift, and few are suitable for navigation except in their lower reaches . Although most rivers are less than 300 kilometers in length, their rapid flow from the mountains provides a valuable, renewable resource: hydroelectric power generation . Japan's hydroelectric power potential has been exploited almost to capacity . Seasonal variations in flow have led to extensive development of flood control measures . Most of the rivers are very short . The longest, the Shinano River, which winds through Nagano Prefecture to Niigata Prefecture and flows into the Sea of Japan, is only 367 kilometers long . The largest freshwater lake is Lake Biwa, northeast of Kyoto . </P> <P> Extensive coastal shipping, especially around the Seto Inland Sea (Seto Naikai), compensates for the lack of navigable rivers . The Pacific coastline south of Tokyo is characterized by long, narrow, gradually shallowing inlets produced by sedimentation, which has created many natural harbors . The Pacific coastline north of Tokyo, the coast of Hokkaidō, and the Sea of Japan coast are generally unindented, with few natural harbors . </P> <P> In November 2008, Japan filed a request to expand its claimed continental shelf . In April 2012, the U.N. Commission on the Limits of the Continental Shelf recognized around 310,000 square kilometres (120,000 sq mi) of seabed around Okinotorishima, giving Japan priority over access to seabed resources in nearby areas . According to U.N. Commission on the Limits of the Continental Shelf, the approved expansion is equal to about 82% of Japan's total land area . The People's Republic of China and South Korea have opposed Japan's claim because they view Okinotorishima not as an island, but as a group of rocks . </P>

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