Rivers are generally steep and swift, and few are suitable for navigation except in their lower reaches. Most rivers are fewer than 300 kilometers in length, but 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, 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.