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Show Chapter 2 The River and Its Tributaries The Allegheny and Monongahela Rivers join at Pittsburgh to form the Ohio River. From Pitts- burgh, the Ohio flows generally southwest for 981 miles to Cairo, 111., where it joins the Mississippi River. This river is the largest tributary in the Mississippi River system in volume of flow and is second only to the Missouri in size of drainage area. Average an- nual flow of the Ohio, including the Tennessee, during the period 1930 through 1941, was 231,000 cubic feet per second (168,630,000 acre-feet) near the mouth-more than 3J/2 times the amount of water the Missouri River empties into the Mississippi. The valley floor of the main river is narrow, varying from an average width of less than one- half mile in the reach between Pittsburgh and Wheeling, W, Va., to slightly more than a mile in the Cincinnati-Louisville reach. Although the mountains rise to more than 2,500 feet in Pennsyl- vania, the total fall of the main stem between Pitts- burgh and Cairo, 111., is 429 feet. The Miami drops 600 feet in about 160 miles in Ohio. Within the Ohio Basin there are three major groups of secondary drainage systems-the upper river and its tributaries, the Monongahela and the Allegheny Rivers; the northern tributaries, prin- cipally the Beaver, Muskingum, Scioto, Miami, and Wabash; and the southern tributaries, of which the Kanawha, Big Sandy, Licking, Kentucky, Green, Cumberland, and Tennessee (discussed in a sepa- rate report) are the most important. The Monon- gahela and Allegheny rise in the mountainous sec- tion which borders the basin to the east. Precipi- tous slopes, shallow soils, and heavy precipitation result in high rates of runoff. The northern tribu- taries drain a relatively level glaciated area with deep soils. Here flood plains are wider and the rivers meander over flat valley floors. The southern tributaries rise in mountainous areas and drain hilly uplands in their middle and lower reaches. The basin lies directly in the path usually fol- lowed by cyclonic storms as they move from west to east across the continent in the winter and early spring months. For this reason the watershed fre- quently receives heavy rainfall during the months of January to May, when infiltration, transpiration, and evaporation are at a minimum, and rainfall- runoff relations are greatest. This produces large stream flows during this season and causes general floods on the main stem. Rain falling on snow, which often covers the northern and eastern section of the basin in winter and early spring, results at times in runoff volumes that exceed the amount of rain. On the tributaries there is no well-defined flood season, and damaging floods may occur during any time of the year. The most severe floods on smaller streams are caused by local rains of cloudburst intensity. High floods have swept down the Ohio River and its tributaries from time to time since the days of early settlement. This river has always been a major contributor to floods on the lower Missis- sippi. The largest floods of modern record on the Ohio River occurred in March 1913, March 1936, and January-February 1937. The river has a wide range in stage between low water and extreme high water. At Cincinnati, the extreme high stage recorded is 80 feet-28 feet above the stage at which flood damage starts. Major floods at any given locality are infrequent but overbank stages occur rather often. Floods at Pittsburgh have averaged one in each 10 months 633 |
