The Rhine is a major river in Europe, and it has several tributaries. The Ruhr is a 4-letter tributary of the Rhine located in western Germany. It is the most heavily populated river basin in Europe. The AAR is a 3-letter tributary of the Rhine located in Switzerland. It is the longest river that both rises and ends entirely within Switzerland. The AARE is a 4-letter tributary of the Rhine located in Switzerland. It is the second longest river in Switzerland and is an important source of water for the country.