The answer to this question would seem pretty straight forward. The river is the border between Kentucky and Indiana. Each side is half responsible for any state owned transportation facility across the mighty river. Right? Well, not really.
The actual answer stems back to prior to the creation of the Northwest Territory that would eventually create the states of Illinois, Indiana, Ohio, Michigan and Wisconsin (and part of Minnesota). Historically, the Virginia colony believed that, basically, everything west of the other twelve colonies belonged to them. For instance, what is now Indiana was originally considered part of Illinois County, Virginia. This belief would cause problems for all of the Northwest Territories and for the state of Pennsylvania. The part of Pennsylvania where my family comes from was contested between Virginia and Pennsylvania. To the point where Virginia sent a Lt. Colonel to the area of what is now Allegheny and Westmoreland Counties to assert their interests in the territory. That Lt. Colonel was George Washington.
The area around what is now Clarksville was originally put in place by the Commonwealth of Virginia as payment to Revolutionary War veterans, most famously George Rogers Clark.
When Kentucky County, Virginia, sought entrance into the Union, the border was located not at the mid point of the Ohio and Mississippi Rivers, but at the low water point opposite Kentucky’s territory. This would later be confirmed by the US Supreme Court when Indiana argued the point, and lost. This would create an area south of Evansville that belongs to Kentucky, but is north of the Ohio River, as shown in the below Google Map image. As an aside, for those wondering why the riverboat casinos in southern Indiana never left the pier, it had everything to do with this border. Those boats could hardly turn around before entering Kentucky…a state that, ironically, banned gambling (or, at least casino gambling…horse racing was not included in this ban).
Today, there are mutual agreements when it comes to paying for crossing the Ohio River. Most of the time, it is mostly an even split between Indiana and Kentucky when it comes to finances. Now, one most also include the Federal funding (usually around 80%) that goes into such projects. For instance, with the new Louisville crossings, one was paid for by Indiana, the other by Kentucky. This is in contrast to the fact that the state line is very close to the Indiana side of the river. Tolls collected on these new crossings go to a joint fund to maintain these new bridges. It doesn’t directly benefit either state…just the bridges.
Why does Indiana agree to paying for half a bridge that is only roughly 10% (or even less) in Indiana? It comes down to mutual benefit. Both sides of the river benefit to the building of a crossing. As such crossings become more and more expensive, the shared cost becomes a matter of necessity. So, the next time you cross the Ohio River, remember that you will be in Kentucky a lot sooner than you would think. The other states that border the rivers and Kentucky also deal with the same situation.
The other river border in Indiana, that of the Wabash between Indiana and Illinois, is less contentious. The state line runs down the center of the old channel of the Wabash. Due to the river moving over the years, it leads to some parts of each state on the wrong side of the current river.