The northwestern boundary of the State of New York is formed by Lake Ontario, of which the St. Lawrence River is the outlet, flowing northeastward into Canada. Ontario is the smallest and the lowest in level of the group of Great Lakes, its name given by the Indians meaning the "beautiful water." It is about one hundred and eighty miles long, and its surface is two hundred and thirty-one feet above tide, but it is fully five hundred feet deep, so that it has more depth below the ocean level than the lake surface is above. It has a marked feature along its southern shore, where a narrow elevation known as the "Lake Ridge" extends nearly parallel with the edge of the lake, and from four to eight miles distant. The height of this ridge usually exceeds one hundred and sixty feet above the lake level, and in some places is nearly two hundred feet, and it is, throughout, from five to twenty feet above the immediate surface of the land, there being a width at the summit of some thirty feet, from which the ground slopes away on both sides. This ridge is regarded as an ancient shore-line formed by the waters of the lake, and the chief public highway on the southern side of the lake is laid for many miles along its summit. The main tributaries of Ontario from New York are the Black, Oswego and Genesee Rivers. The Black River gathers various streams draining the western slopes of the Adirondacks, and its name comes from the dark amber hue of the waters. It flows northwest through a forest-covered region, pours down Lyons Falls, a fine cataract of seventy feet, passes the manufacturing towns of Lowville and Watertown, and finally discharges by the broadened estuary of Black River Bay into the east end of Lake Ontario. From Rome, on the Mohawk, a canal is constructed northward to the Black River.