Lake levels are referenced to sea level, but what exactly is sea level? The planet's oceans are not all the same height, for instance the absolute height of the Pacific Ocean is higher than the Atlantic Ocean, so it is important to establish exactly what "sea" we are using as a baseline reference. For the Lake of the Ozarks "sea level" was established by the "Mean Gulf Level" from Biloxi Mississippi.
The Gulf of Mexico is a large sea, typically differentiated from an ocean by being near and adjoining land, usually partially enclosed by it. The gulf is certainly a large enough body of water to be influenced by tides which means establishing a single average (mean) level must be done by tidal stations, distributed over a large area of coastline. Measurements at these stations are made using vertical benchmarks positioned on land and therefore must take into consideration that the land itself also moves vertically over time. Globally this effect can be quite pronounced as, for example, the coastline of the Gulf of Mexico is typically in a state of subsidence (sinking), effectively raising sea levels, while the Gulf of Alaska is rising, lowering sea levels, due to less weight on the land from melting glaciers (isostatic rebound).
The "Mean Gulf Level" was established by the Mississippi River Commission near the end of the 19th century, well before Bagnell Dam's construction. As time goes by, and the relative sea level changes due to geological processes and rising sea levels due to
* 2017 Blogger's Note: I see no point in trying to appease the climate change deniers, least of all by mincing words. It may be climate change in a general sense of the topic, but the actual trend is for a warmer planet, and that is the cause of rising sea levels. The world might be a better place if we stop getting our opinions from social media (this blog included), news outlets, or politicians. Try understanding at least some of the science of a warming planet for yourself rather than just pointing out that it still snows in the winter as evidence to the contrary. #thisshitisgettingold