The valley referred to, flooded by the back water from the dam, affords an excellent basin at the entrance of the canal, free from the influence of the river current, and the latter forms a natural, ready-made canal, 3300´ long, needing only slight excavation on the points of two or three spurs for rectifying the channel. From the head of this valley, a canal 1·82 miles long extends across a broken country of moderate elevation, intersecting one deep narrow ravine, debouching towards the San Juan, across which a short embankment will be necessary, and enters the valley of the river San Francisco. This river San Francisco flows east, north-east, and east, approximately parallel to the San Juan, and separated from it by a range of hills to a point about nine miles (in straight line) from the dam, then, receiving a considerable tributary (the Cano de los Chanchos) from the north-east, turns abruptly to the south-east and south, and enters the San Juan. Its valley thus forms an irregular flattened Y, with its foot or stem resting on the San Juan, one arm extending westerly to within a short distance of the dam, and the other easterly in the direction of Greytown. Across the stem of this Y, just below the junction of the two arms, will be built an embankment 6500 feet long on the crest, and having a maximum height of 51 feet. This embankment will retain the water of the San Francisco and its tributaries, flooding the whole upper valley (the arms of the Y) to a depth of from 30 to 50 feet, and forming a large lake at the same level as the river above the dam—in other words, a continuation of the summit level. Proceeding from the end of the short canal already described, the main canal passes down the westerly arm of this broad, deep, crescent-shaped basin, past the embankment, then up the easterly arm to the western foot of the divide between the San Francisco and the San Juanillo, 12·55 miles from the dam, and within 19·48 miles of Greytown. Here the eastern division of the canal is entered, beginning at the Saltos de Elvira, whence it proceeds nearly due east, through the broad, flat upper valley of the Arroyo de las Cascadas, cutting a spur here and there to the “divide,” less than one mile from the Saltos, and 280 feet above the sea. Then curving gradually to the south-east, across the little plain at the summit, it cuts a steep, narrow spur, enters the valley of the Deseado, a stream flowing into the San Juanillo, follows its bed a short distance, then crosses to the left bank, and reaches the site of the upper lock of the eastern flight, 14,200 feet from the Saltos. The average cut for this distance is 149 feet.