Generally the main river and tributary are easily identified, as one stream is both longer and carries more water than the other. Occasionally one stream is longer, but the other carries more water. This case offers no fixed rules, but the longer valley is usually chosen as the main valley. In Switzerland the long Alpine Rhine is chosen as the main stream, although the Aar carries more water. The Mississippi River carries more water while the Missouri River is much longer, but is still rated the tributary.