Interpreting NOAA Hydrographs

Graph published with permission from the NOAA

In its simplest form, you are looking at the river level taken at the upriver side of the McAlpine Locks and Dam, located just downriver from downtown Louisville. The normal level, or "pool," is approximately 12 feet, and is maintained by the U.S. Army Corps of Engineers. Past data is charted along with future predictions, which can change.

As water flows into the Ohio River from precipitation in the Ohio Valley area, the levels will change as the Corps adjusts upriver dams to control flooding and maintain a level suitable for safe navigation. As the river goes up past 14 feet, you will see a proportionate increase in flow, as indicated on the right side of the graph measured in thousands of cubic feet per second.

At 19 feet, several of the local marina parking lots are flooded, and flooding along River Road starts to occur at 23 feet. Most marinas have floating docks, but your boat may be affected by changing levels if you are connected to a fixed structure. This makes it especially important to check levels frequently.

If you pay attention to the weather, you will note that river levels vary depending on how much rain the Ohio Valley received in the preceding days. Small showers may have an insignificant effect, while the remnants of a tropic depression moving from Louisville to Cincinnati and northeast to Pittsburgh can affect river levels and the debris field for over a week.

While powerboaters aren't affected as much by the flow at normal levels, sailboaters will point out that even at a 12' pool, there may be significant current. This can be indicated after a significant rainfall where McAlpine is maintained at 12', but the Markland Upper is showing higher than normal. Therefore, looking at the McAlpine Upper numbers only provide part of the equation.

Another interesting feature is the "hump" in the river. That is, depending on the flow from upriver, there can be as much as several feet of difference in levels between the McAlpine Upper reading and what you see upriver. For example, the McAlpine Upper reading could indicate normal levels, while somebody at Westport could see their fixed dock underwater. This is a factor of the amount of water being dumped into the McAlpine pool.

It is interesting to note that prior to the installation of locks and dams, the river was largely unnavigable due to low levels and rapids. Westport, KY, as a matter of fact, was central to the Underground Railroad, as people could walk across the Ohio River there during low levels in the summer.

-Captain Eric