Page:Popular Science Monthly Volume 45.djvu/201

Rh broken surface whose summits represent the once comparatively level area. At Parker City, Pa., an elevator was once used to lift foot travelers from the lower terrace to the upper terrace, two hundred feet above.

Although flowing in so deep a trough, the present Ohio River is considerably elevated above the ancient bottom. This is owing to the fact that during the Glacial period such an excessive amount of gravel was brought down from the Alleghany River and other northern tributaries that the old channel was silted up to a considerable depth. At Cincinnati there is more than one hundred feet of gravel between the present river bottom and the rock bottom. Below the mouths of the most important northern tributaries the accumulations were much greater than this. At Cincinnati the channel was choked with gravel from the Little Miami to a height of one hundred and twenty feet above the present river. Subsequently this was partly eroded away, leaving the one-hundred-and-twenty-foot gravel terrace which is now occupied by Fourth Street.

It is fortunate for civilization that there are left along the trough of the Ohio numerous remnants of this high-level glacial terrace; otherwise the cities would be even more subject to damage from floods than they are now; for the Ohio River is subject to greater fluctuations of level than almost any other stream in the world. During the flood of 1884 the water rose at Cincinnati seventy-one feet, submerging the railroad stations and much of the lower part of the city, but leaving that portion which was upon the glacial terrace fifty feet above water. The cities which were not favored with so marked a gravel terrace, or had not taken advantage of their opportunities, were for many days turned into miniature Venices, the lower stories of the houses being generally submerged by the muddy torrent, and boats being able to pass freely through all the streets.

The cause of these enormous floods along the Ohio is readily perceived; for, as already remarked, the slope of the streams rising along the summit of the Alleghany Mountains and flowing into the Ohio is so rapid that the water from the rains and melting snows finds its way into the main trough of the river in an incredibly short time, while the trough is so narrow in places, especially just below Cincinnati, as greatly to impede the progress of the current. Two or three inches of rainfall over two hundred thousand square miles provides an enormous quantity of water, which, upon being suddenly transferred to the river channel, turns a stream which can sometimes be forded in dry weather into a steadily advancing column of water one thousand miles long and from fifty to seventy-five feet deep. It is interesting to watch from the weather bulletins the progress of the waves that move down the