The Asian rivers that are important highways of commerce are few in number. The Amur, Yangtze, Indus, and Cambodia have each considerable local commerce. The Hugli, a channel in the delta of the Ganges, has a channel deep enough for ocean steamships. The tributaries of the Lena, Yenisei, and Ob have been of the greatest service in the commercial development of northern Asia from the fact that their valleys are both level and fertile. Because of a high interior and abrupt slopes, the rivers of Africa are not suitable for navigation to any considerable extent; the channels are uncertain and the rivers are interrupted by rapids. The Nile has an occasional steamboat service as far as the "First Cataract," but in high water the service is sometimes extended farther. The Kongo has a long stretch of navigable water, but is interrupted by rapids below Stanley Pool. Similar conditions obtain in the Zambezi. The lower part of the Senegal affords good navigation. The Niger has in many respects greater commercial possibilities than other rivers of Africa. It is navigable to a distance of three hundred miles. =Canals.=--Canals easily rank among the most important means of traffic, as a rule, supplementing other navigable waters. Thus, by means of an elaborate system of canals, goods are transferred by water, from one river-basin to another, so that practically all the navigable streams of western Europe are connected. Canals are extensively used to avoid the falls or rapids that separate the various reaches of rivers. The water itself by means of locks lifts the boat to a higher level or transfers it to a lower reach, thus saving the expense of unloading, transferring, and reloading a cargo. The manner in which canals supplement the obstructed navigation of a river is seen in the case of the St. Lawrence. This river is obstructed in several places by rapids, but by means of canals steamship service connects the Great Lakes, not only with Quebec, but with ports of the Mediterranean Sea as well; indeed, it is possible to send a cargo from Duluth, at the head of Lake Superior, to Odessa or Batum, on the shores of the Black Sea. The internal water-ways of Canada have been splendidly developed. The Canadian St. Marys Canal furnishes an outlet to Lake Superior for vessels drawing twenty-one feet. The Welland Canal connects Lakes Erie and Ontario. The Rideau Canal and River connect Kingston and Lake Ontario with the Ottawa, and the latter with its canals is navigable to the St. Lawrence. With a population of less than six millions the Dominion Government has spent nearly one hundred million dollars in the improvement of internal water-ways. [Illustration: PROFILE OF ERIE CANAL HORIZONTAL SCALE 100 MILES TO THE INCH, VERTICAL SCALE 1,000 FEET TO THE INCH]