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Interpreting NOAA Hydrographs

NOAA Hydrograph

Graphs 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 maintain a level suitable for safe navigation. As the river goes up, 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 Lower 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


And now, the rest of the story:

Capt. Skip Davidson (ret.), from the US Army Corps of Engineers, adds the following information:

"Let's talk about Ohio River dam operations and river levels, because there is widespread confusion and misinformation about all of them.

"The dams on the Ohio have zero flood control capabilities; they are navigation dams only. The lockmaster is charged with maintaining his pool elevation. He is not concerned with what happens in the pool below his dam.

"As the river on the upper gauge starts to rise, the lock operator begins too raise the dam gates. If the river continues to rise after the gates are all the way open the dam is said to be "all out", and they have no more control.

"As the river level falls, the dam gates are lowered and the operator tries to maintain the normal pool elevation. In the case of McAlpine the normal pool elevation is 420' above sea level. This equates to a gauge level of 12'.
There is no real good way to determine pool elevation except at a local gauge.

"If McAlpine's upper gauge is at 12' and Markland's lower gauge is at the normal pool gauge reading of 12' then you have a flat pool at 420' elevation--it's a lake that has no current. This rarely happens."


NOAA Hydrograph


"The upper gauge at McAlpine is typically pretty steady but the lower gauge at Markland (or any lower gauge) bounces around because of dam operations. If McAlpine is at 420' and Markland's lower gauge jumps 5' then the water at Markland's lower side is at elevation 425' which introduces a slope to the river creating a current. A straight line slope is commonly assumed. The greater the disparity of the gauges the faster the current runs. So we have Markland lower at 5' above pool, McAlpine at pool then half way between them we can expect the river to be 2.5' above pool.

"McAlpine's upper gauge is located on the down-stream side of the 2nd St bridge, so the slope between there and Turners is negligible but is a factor at Tartans. "


Do the math:

Using Skip's great explanation, here is an example detailing the flow just a few days before Thunder Over Louisville, 2011:

For starters, check the McAlpine Upper number to see what the level is downtown:

Next, check the Markland Lower guage to see what the river level is 75 miles upstream, at the Tailwater side of the Markland Locks and Dam:


Since both use a datum of approximately 408', you can zero that number out of the calculations for simplicity. Take the difference between those numbers to calculate the slope of the river. This number will give you a good indicator of the current you can expect:

As you can see, in this example the folks at Westport, for example, will see water levels of 6.8' higher than downtown (0.27 ft per mile multiplied by 25 miles). This was consistant with reports from people along the river on the day this example was produced.

 

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Eric Grubb
Founder, Port KY
Licensed Master

Eric grew up around boats, trading summers on board his parents' Sea Rays for many man-hours of swabbing the decks. He grew up by the little town of Sewickley, Pennsylvania, overlooking the the Dashields Locks and Dam. He has traveled the Great Lakes, Lake Huron's North Channel, Gulf of Mexico and several rivers to include the Ohio, Allegheny, Monongahela, Kanawha, Mohawk (Erie Canal), Tennessee, Tombigbee, Black Warrior and Mobile Rivers.

As a commercial pilot, Eric flies jets and is a flight instructor. He has owned recreational boats ranging from PWCs to most recently, a flybridge convertible that he keeps in a Louisville marina (MM 590). You can also find him with his family on the "Escape Pod," an 18' fishing boat. His most memorable journey was aboard the J. S. Lewis, a 155' towboat in service since 1931.

Eric is a USCG Licensed Master with a Commerical Tow Assistance rating, and is a member of the Sons and Daughters of Pioneer Riverman and the Louisville Sail and Power Squadron. After moving to Louisville, he conceived the idea for Louisville's Port KY website while searching for information to help him become a safer and more knowledgable local boater. He has worked hard over the years to educate other boaters by promoting safety classes through Port KY and by hosting captain's classes and related events.

 

 

 

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