Ohio River

==Geography and hydrography==

The combined Allegheny-Ohio river is {{convert|1310|mi}} long and carries the largest volume of water of any tributary of the Mississippi. The Indians and early European explorers and settlers of the region often considered the Allegheny to be part of the Ohio. The [[Point State Park|forks]] (the confluence of the Allegheny and Monongahela rivers at what is now Pittsburgh) were considered a strategic military location by colonial French and British, and later independent American military authorities.

[[File:CairoIL from space annotated.jpg|thumb|left|The [[confluence]] of the [[Mississippi River|Mississippi]] and Ohio rivers is at [[Cairo, Illinois]].]]
The Ohio River is formed by the confluence of the [[Allegheny River|Allegheny]] and [[Monongahela River|Monongahela]] rivers at what is now [[Point State Park]] in [[Pittsburgh]], [[Pennsylvania]]. From there, it flows northwest through [[Allegheny County, Pennsylvania|Allegheny]] and [[Beaver County, Pennsylvania|Beaver]] counties, before making an abrupt turn to the south-southwest at the [[West Virginia]]–Ohio–Pennsylvania triple-state line (near [[East Liverpool, Ohio]]; [[Chester, West Virginia]]; and [[Ohioville, Pennsylvania]]). From there, it forms the border between West Virginia and Ohio, upstream of [[Wheeling, West Virginia]].

[[File:Horseshoe bend in Ohio River.jpg|thumb|The Ohio River as seen from [[Fredonia, Indiana]]]]
The river follows a roughly southwest and then west-northwest course until Cincinnati, before bending to a west-southwest course for most of the remainder of its length. The course forms the northern borders of West Virginia and [[Kentucky]]; and the southern borders of [[Ohio]], [[Indiana]] and [[Illinois]], until it joins the [[Mississippi River]] at the city of [[Cairo, Illinois]]. Where the Ohio joins the Mississippi is the lowest elevation in the state of Illinois, at {{convert|315|ft|m}}.

[[File:Floods Recede around the Wabash-Ohio Confluence.jpg|thumb|left|Natural-color satellite image of the Wabash-Ohio confluence]]
The Mississippi River flows to the Gulf of Mexico on the [[Atlantic Ocean]]. Among rivers wholly or mostly in the United States, the Ohio is the second largest by discharge volume and the tenth longest and has the eighth largest drainage basin. It serves to separate the [[Midwestern United States|Midwestern]] [[Great Lakes]] states from the Upper South states, which were historically [[Border states (American Civil War)|border states]] in the Civil War.

The Ohio River is a left (east) and the largest [[tributary]] by volume of the [[Mississippi River]] in the [[United States]]. At the [[confluence]], the Ohio is considerably bigger than the Mississippi, measured by long-term mean discharge. The Ohio River at Cairo is 281,500&nbsp;cu&nbsp;ft/s (7,960&nbsp;m<sup>3</sup>/s);<ref name=Leeden1990 /> and the Mississippi River at [[Thebes, Illinois]], which is upstream of the confluence, is 208,200&nbsp;cu&nbsp;ft/s (5,897&nbsp;m<sup>3</sup>/s).<ref name=USGS2009>{{cite web |url=http://wdr.water.usgs.gov/wy2009/pdfs/07022000.2009.pdf |title=Water Data Report: 07022000 Mississippi River at Thebes, IL Summary Statistics |publisher=US Geological Survey |access-date=December 19, 2018 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117121824/https://wdr.water.usgs.gov/wy2009/pdfs/07022000.2009.pdf |url-status=live }}</ref> The Ohio River flow is greater than that of the Mississippi River, so hydrologically the Ohio River is the main stream of the river system.

===River depth===
[[File:Lawrenceburg-indiana-from-above.jpg|thumb|[[Lawrenceburg, Indiana]], is one of many towns that use the Ohio as a shipping avenue.]]
The Ohio River is a naturally shallow river that was artificially deepened by a series of [[dam]]s. The natural depth of the river varied from about {{convert|3|to|20|ft|m|0}}. The dams raise the water level and have turned the river largely into a series of [[reservoir]]s, eliminating shallow stretches and allowing for commercial navigation. From its origin to Cincinnati, the average depth is approximately {{convert|15|ft|m|0}}. The largest immediate drop in water level is below the McAlpine Locks and Dam at the [[Falls of the Ohio National Wildlife Conservation Area|Falls of the Ohio]] at [[Louisville, Kentucky]], where flood stage is reached when the water reaches {{convert|23|ft|m|0}} on the lower gauge. However, the river's deepest point is {{convert|168|ft|m|0}} on the western side of Louisville, Kentucky. From Louisville, the river loses depth very gradually until its confluence with the Mississippi at [[Cairo, Illinois]], where it has an approximate depth of {{convert|19|ft|m|0}}.

Water levels for the Ohio River from Smithland Lock and Dam upstream to Pittsburgh are predicted daily by the [[National Oceanic and Atmospheric Administration]]'s Ohio River Forecast Center.<ref>{{cite web |url=https://www.weather.gov/ohrfc/ |title=Ohio RFC |publisher=US Department of Commerce, NOAA, National Weather Service |access-date=March 10, 2017 |archive-date=March 12, 2017 |archive-url=https://web.archive.org/web/20170312072122/https://www.weather.gov/ohrfc/ |url-status=live}}</ref> The water depth predictions are relative to each local flood plain based upon predicted rainfall in the Ohio River basin in five reports as follows:

* [[Pittsburgh|Pittsburgh, Pennsylvania]], to [[Hannibal Locks and Dam]], Ohio (including the Allegheny and Monongahela rivers)
* [[Willow Island Locks and Dam]], Ohio, to [[Greenup Lock and Dam]], Kentucky (including the Kanawha River)
* [[Portsmouth, Ohio]], to [[Markland Locks and Dam]], Kentucky
* [[McAlpine Locks and Dam]], Kentucky, to [[Cannelton Locks and Dam]], Indiana
* [[Newburgh Lock and Dam]], Indiana, to [[Golconda, Illinois]]
The water levels for the Ohio River from Smithland Lock and Dam to Cairo, Illinois, are predicted by the [[National Oceanic and Atmospheric Administration]]'s Lower Mississippi River Forecast Center.<ref>{{cite web |url=http://www.weather.gov/lmrfc/ |title=Lower Mississippi RFC |publisher=US Department of Commerce, NOAA, National Weather Service |access-date=March 10, 2017 |archive-date=February 28, 2017 |archive-url=https://web.archive.org/web/20170228181631/http://www.weather.gov/lmrfc/ |url-status=live}}</ref>
* [[Smithland Lock and Dam]], Illinois, to [[Cairo, Illinois]]

{{wide image|PennsylvaniaRailroadBridgePanorama.jpg|5000px|Panorama of the Ohio at its widest point, just west of downtown Louisville, Kentucky|700px|center}}

===List of major tributaries===
The largest tributaries of the Ohio by discharge volume are:
{{Columns-start}}
*Tennessee River {{convert|70575|cuft|abbr=on}}/sec
*Cumberland River {{convert|37250|cuft|abbr=on}}/sec
*Wabash River {{convert|35350|cuft|abbr=on}}/sec
*Allegheny River {{convert|19750|cuft|abbr=on}}/sec
*Kanawha River {{convert|15240|cuft|abbr=on}}/sec
{{Column}}
*Green River {{convert|14574|cuft|abbr=on}}/sec
*Monongahela River {{convert|12650|cuft|abbr=on}}/sec
*Kentucky River {{convert|10064|cuft|abbr=on}}/sec
*Muskingum River {{convert|8973|cuft|abbr=on}}/sec
*Scioto River {{convert|6674|cuft|abbr=on}}/sec
{{columns-end}}

By drainage basin area, the largest tributaries are:<ref name="orsanco">{{cite web |title=Tributaries |url=http://www.orsanco.org/river-facts/tributaries/ |website=www.orsanco.org |access-date=March 13, 2019 |archive-date=March 18, 2019 |archive-url=https://web.archive.org/web/20190318160952/http://www.orsanco.org/river-facts/tributaries/ |url-status=live}}</ref>
{{Columns-start}}
*Tennessee River {{convert|40910|mi2|abbr=on}}
*Wabash River {{convert|33100|mi2|abbr=on}}
*Cumberland River {{convert|17920|mi2|abbr=on}}
*Kanawha River {{convert|12200|mi2|abbr=on}}
*Allegheny River {{convert|11700|mi2|abbr=on}}
{{column}}
*Green River {{convert|9230|mi2|abbr=on}}
*Muskingum River {{convert|8040|mi2|abbr=on}}
*Monongahela River {{convert|7400|mi2|abbr=on}}
*Kentucky River {{convert|6970|mi2|abbr=on}}
*Scioto River {{convert|6510|mi2|abbr=on}}
{{Columns-end}}

The largest tributaries by length are:<ref name="orsanco" />
{{Columns-start}}
*Cumberland River {{convert|693|mi|abbr=on}}
*Tennessee River {{convert|652|mi|abbr=on}}
*Wabash River {{convert|474|mi|abbr=on}}
*Green River {{convert|370|mi|abbr=on}}
*Allegheny River {{convert|325|mi|abbr=on}}
{{column}}
*Licking River {{convert|320|mi|abbr=on}}
*Kentucky River {{convert|255|mi|abbr=on}}
*Scioto River {{convert|237|mi|abbr=on}}
*Great Miami River{{convert|161|mi|abbr=on}}
*Little Kanawha River {{convert|160|mi|abbr=on}}
{{Columns-end}}

Major tributaries, in order from the headwaters, include:<ref name="orsanco" />
{{Columns-start}}
* [[Allegheny River]] – Pittsburgh, Pennsylvania
* [[Monongahela River]] – Pittsburgh
* [[Beaver River (Pennsylvania)|Beaver River]] – [[Rochester, Pennsylvania]]
* [[Little Muskingum River]] - Ohio
* [[Muskingum River]] – Marietta, Ohio
* [[Little Kanawha River]] – [[Parkersburg, West Virginia]]
* [[Hocking River]] – [[Hockingport, Ohio]]
* [[Kanawha River]] – [[Point Pleasant, West Virginia]]
* [[Guyandotte River]] – [[Huntington, West Virginia]]
* [[Big Sandy River (Ohio River)|Big Sandy River]] – Kentucky-West Virginia border
* [[Little Sandy River (Kentucky)|Little Sandy River]] – [[Greenup, Kentucky]]
* [[Little Scioto River (Ohio River)|Little Scioto River]] – [[Sciotoville, Ohio]]
{{column}}
* [[Scioto River]] – [[Portsmouth, Ohio]]
* [[Little Miami River]] – [[Cincinnati|Cincinnati, Ohio]]
* [[Licking River (Kentucky)|Licking River]] – [[Newport, Kentucky|Newport]]-[[Covington, Kentucky|Covington]], Kentucky
* [[Great Miami River]] – Ohio-Indiana border
* [[Kentucky River]] – [[Carrollton, Kentucky]]
* [[Salt River (Kentucky)|Salt River]] – [[West Point, Kentucky]]
* [[Green River (Kentucky)|Green River]] – near [[Henderson, Kentucky]]
* [[Wabash River]] – Indiana-Illinois-Kentucky border
* [[Saline River (Illinois)|Saline River]] – Illinois
* [[Cumberland River]] – [[Smithland, Kentucky]]
* [[Tennessee River]] – [[Paducah, Kentucky]]
* [[Cache River (Illinois)|Cache River]] – Illinois
{{Columns-end}}

===Drainage basin===
The Ohio's drainage basin covers {{convert|189422|sqmi}}, encompassing the easternmost regions of the [[Mississippi Basin]]. The Ohio drains parts of 14 states in four regions.

* Northeast
** [[New York (state)|New York]]: a small area of the southern border along the headwaters of the Allegheny.
** [[Pennsylvania]]: a corridor from the southwestern corner to the north-central border.
* Mid-Atlantic/Upper South
** [[Maryland]]: a small corridor along the [[Youghiogheny River]] on the western border.
** [[West Virginia]]: all but the [[Eastern Panhandle of West Virginia|Eastern Panhandle]].
** [[Kentucky]]: all but a small part in the [[Jackson Purchase|extreme west]] drained directly by the Mississippi.
** [[Tennessee]]: all but a small part in the extreme west drained directly by the Mississippi, and a very small area in the southeastern corner which is drained by the [[Conasauga River]].
** [[Virginia]]: most of southwest Virginia.
** [[North Carolina]]: the western quarter.
* Midwest
** [[Ohio]]: 80% (all except a northern strip bordering Lake Erie, and the northwest corner)
** [[Indiana]]: all but the northern area.
** [[Illinois]]: the southeast quarter.
* Deep South
** [[Georgia (U.S. state)|Georgia]]: the far northwest corner.
** [[Alabama]]: the northern portion.
** [[Mississippi]]: the northeast corner.