Only a few rivers have a steady flow throughout the year at a depth sufficient to carry large boats. On most streams destructive floods at certain seasons and low waters at others interfere with navigation during a considerable part of the year. Most rivers have sand-bars, sunken rocks or logs in the channel, making the passage of boats difficult and dangerous. Others are well suited for navigation, except at points where rapids and falls make it impossible for boats to pass. The Ohio, the Tennessee, the Missouri and the upper Mississippi abound in such dangerous places and these should be canalized. It is the improving of rivers in these ways, dredging harbors to make them safer, and digging canals to provide a short passage between two bodies of water, that constitute what is known as the Improvement of Inland Waters. If you look at a map showing the navigable streams of the United States you will see that nearly all of them lie in the eastern part. The Mississippi is like a great artery with branches extending in all directions, east and west. The Great Lakes, with their outlet, the St. Lawrence River, and the many important rivers emptying into the Atlantic Ocean and the Gulf of Mexico, such as the Merrimac, Hudson, Delaware, Susquehanna, Potomac and Rio Grande, form great highways for all the commerce of the eastern part of the country, while the Columbia, Sacramento and Colorado Rivers, with their branches, are the only navigable streams of any importance west of the Mississippi River system. In some places a small portion of land divides two important water areas, and canals dug through this neck of land change the commercial routes of the whole world. Such are the Isthmus of Suez, eighty-seven miles wide, through which a canal was cut that saves a sailing distance of 3,700 miles from England to India. Only the Isthmus of Panama, forty-nine miles in width, divides the Atlantic from the Pacific Ocean. When the canal across this narrow strip is completed, the sailing distance from New York to San Francisco will be shortened 8,000 miles, the entire distance around South America. The Sault Ste. Marie Canal, connecting Lakes Superior and Huron, is only a little more than a mile and a half long, but it opens up the entire iron, copper, lumber and wheat resources of the Northwest to cheap water passage through the other lakes to the manufacturing region of the East. The Erie Canal, by connecting Lake Erie with the Hudson River from Buffalo to Albany, New York, makes the only water passage from the Great Lakes to the ocean that lies within the borders of the United States.