A few more words about the main physical features of the isthmus are necessary for the reader to understand the nature of the problem. The two most important factors in the problem, as we have seen, are, firstly, the river Chagres with its tributaries, the Trinidad, Gatun, and twenty others; and, secondly, the range of low hills on the Pacific side through which any canal from Colon to Panama must pass. The river Chagres is a great mountain torrent which enters the Caribbean Sea a little west of Colon. The canal follows its course inland for about 26 miles, when the river valley turns sharply north-east and the canal continues straight on to the Pacific. The Chagres is not a river to be despised. The rainfall on the isthmus is very heavy, especially on the Atlantic side, where 140 inches per annum have been recorded. The isthmian rivers are all liable to quickly-swelling floods, the Chagres at Gamboa having been known to rise 35-1/2 feet in twenty-four hours. The two different types of canal involve equally different methods of dealing with this formidable stream. It must either be harnessed to the work or firmly and finally shut off from any interference with the canal. De Lesseps, who had chosen the tide-level scheme, proposed to turn the Chagres and other rivers into diversion channels, so that they could get safely to the sea without crossing the line of the canal or having any connection with it. This would have involved a work of excavation and construction scarcely less gigantic than the building of the canal itself.