2 GREAT Web Resources for East Coast Boaters

[PSG-1]

Here's a couple of links I have in my frequently visited sites:


https://www.srh.noaa.gov/serfc/

This is the Southeast River Forecast Center. It shows a map of the southeastern US (you can zoom out, and zoom back in for anywhere else in the US, as well) You will notice a bunch of little green, yellow, orange, red, or purple squares. Each one of these is a guaging station along a river, as well as some tide guages on the ocean. If you move your cursor over these, you will get a small tab that shows the current state of that river, at a glance. If you click on that square, it will take you to that particular guage, where you can see other data, like historical high and low levels, as well as the effects of flooding, from 'action stage' all the way up to 'major' Some of them even have photos of that location under different flows. VERY good info for the river boaters out there!




Next one:

https://www.charts.noaa.gov/OnLineViewer/AtlanticCoastViewerTable.shtml

This link will let you take a look at any NOAA chart on the east coast from Maine to Florida, and then some. You can zoom in on any of them for a closer look, and move the zoom window around to move the chart around. (Still haven't figured out how to zoom back out, LOL) Anyhow, this is another good resource for the marine boaters who run the ICW, the inlets, and saltwater areas.

 

[Lowe 2070 CC]

Always look to see what the gauge "0" for the river in question, usually on the bottom left of the chart.

Gauge "0" is the surface elevation above mean sea level (msl) that the gauge references. In my case, gauge "0" for the Mississippi River at Vicksburg is 46.23' above msl. Thus a river stage of 6.6' today means that the surface of the river is 52.83' (46.23' + 6.6') above msl.

These gauges do not give depth of water, just surface elevation.

Around here falling river means catching fish, 27' and falling means lots of fish.

Record high at Vicksburg was 57.1' in May of 2011 (103.33' above msl).
Record low was -7.00 in 1940 (39.23' above msl).

The level here routinely varies 30-40' from high to low water in most years. Lots of folks unfamiliar with the lower Mississippi don't realize how much it moves in a normal year.

 

[PSG-1]

Lowe, in some instances, the guages are based on height above mean sea level, but in some instances, they ARE based on water depth. And sometimes, both occurring on the same river!

For instance, there are 2 guages on the Congaree River in SC, one at Carolina Eastman, which generally varies between 105 and 120 ft. This is the height above sea level. But the next guage upstream, at Columbia, ranges from 5 to 20 ft. This is water depth, not reading above sea level. On the Great PeeDee River, there's a guage at Bennettsville, it runs between 50 and 75 ft. This is the height above sea level. The next guage upstream is at Cheraw, and this ranges between 2 and 30 ft, which is the depth, not the elevation, because the river at Cheraw is about 80-90 ft above sea level.

So, when referencing a guage, check to see what its "0" reading is, it's always shown on the site. Sometimes it will be "0" and when it is, you know this is the water depth. If it shows anything above 0, then you know it's the elevation of that part of the river above sea level.

 

[Lowe 2070 CC]

Gauge "0" at Carolina Eastman is "0" msl, so the reading is always the surface elevation, 108.4' + 0' = 108.4' above msl.

The Gauge "0" at Columbia is 113.02'. So the reading of 4.6' today means the surface elevation of the Congaree River at Columbia is 113.02' + 4.6' = 117.62' above msl. 4.6' is not the depth.

The PeeDee at Bennetsville has a Gauge "0" of "0", like the Congaree at Carolina Eastman. Thus todays reading of 57.6' is actually 57.6' + 0 = 57.6' above msl surface elevation.

The PeeDee at Cheraw has a gauge "0" of 56.92'. Thus todays reading of 4.1' is actually 56.92' + 4.1' = 61.02' above msl. Not 4.1' deep.

I will reiterate,

"Always look to see what the gauge "0" for the river in question, usually on the bottom left of the chart.

Gauge "0" is the surface elevation above mean sea level (msl) that the gauge references. "

 

[PSG-1]

Thanks for clarifying that! I was apparently mistaken about the readings.

I am curious, though, is there any rhyme or reason for how they base the datum? I'm wondering why some stations are based on "0" + elevation, and some are based on just the elevation.

 

[Lowe 2070 CC]

Sometimes yes, sometimes no. These "arbitrary" elevations are typically tied to NGVD29 (National Geodetic Vertical Datum of 1929) or NAVD88 (North American Vertical Datum of 1988) benchmarks, which is why I caution folks to always check the gauge "0" elevation. It's the only way to make sense of what you are seeing.

Lots of folks confuse the stage with depth. I've worked on a couple of lakes when at public hearings we were addressing a lake level of 71', and a resident began berating us, that he knew every deep hole in the lake and none of them were over 40' deep, which led us to the same discussion about stages.

Most lakes, rivers and reservoirs post their stage as a function of msl. It's the only constant they all share in common. More importantly it allows you to follow topo contour lines to determine flood boundaries at any given level.

The closer you get to the coast the more prevelant 0' becomes, as folks there are quite aware of absolute sea level, although reservoirs and impoundments often adopt their own gauge "0".

During a headwater flood this allows you to see the wall of water moving downstream.

The attached slides are from a presentation discussing the problems inherent in compare flow rates from streams using different datum sources.

 

[PSG-1]

Very good info, that does explain it and clear up the confusion. 8)

I think I see what you mean about being able to track the wall of water as it moves downstream. For instance, when the Pee Dee River is flowing from Blewett Falls under normal conditions, whatever the guage says at Rockingham, it doubles at Cheraw, and then quadruples at 76/301 bridge at PeeDee. I'm guessing this is because of the drop in elevation of the river bed as it runs downstream?

 

[Lowe 2070 CC]

You'll see that the reservoir elevation at Blewett Falls is roughly 50' above the river surface elevation downstream at Rockingham. It then falls another 60' in Elevation to Cheraw and drops another 30' to 76/301. That's 140' in elevation change 90' over an 88 river mile run, or about 1'/mile.

In contrast, Steele Bayou here in the lower delta drops 4.1' over a 32 mile run. 1.5 Inches of drop per mile.

When thinking of a headwater flood on a river, imagine a standing wave moving downstream, such that the elvation only decreases to the degree that it can spread out over the flood plain. The narrower the floodplain and steeper the fall, the higher the wave.