Harbor Water Levels

[Joe Dolan]

This confused me...

I was out in the harbor yesterday.  Water levels were low (around 5 ft lower than my normal charts), but then I noticed that the water levels were on par with my normal charts when I was near the channels or any of the gaps into the lake.  Also, they were the same when I was out past the wall.  What would cause this?  I have used these charts out there for the past bunch of years and do not recall seeing inconsistencies in the maps versus what I am actually seeing?

[John Jarosz]

When you mention "charts", what charts are you looking at? Are they the preloaded charts on your graph? 

When you say "Water levels", is that really water depth?
Here's my understanding of it, which may or may not be on track.

Depths shown on NOAA charts are not the actual depths you show when you turn on your graph recorder. The depths on your graph are the distance from your boat to the bottom.

The depths shown on charts are usually from "Low Water Datum"

I checked this site tidesandcurrents.noaa.gov/stationhome.html?id=9063020 

and it states that water level as of today is 18.2" above "Low Water Datum." If the chart on your graph shows a depth of 20', the actual depth is about a foot & a half more, so your graph will read 21.5' (I think).

I remember when we were getting the FAS's installed by the Darling Co. They needed certain clearances from the surface & the supervisor checked the lake Erie water level every day befor dumping rocks.

To add more confusion, Low Water Datum is given Above Sea Level, since that is the starting point.

Right now Low Water Datum is 569.23 ft above sea level
If any of that makes sense to you, Joe & it fits with what you're seeing, can you explain to me what I just said so I can understand it? 

[Joe Dolan]

I was lost before you even replied, John!  🤣

Here are my definitions:

- Charts - I was viewing my maps through my Humminbird Helix that shows the depth data using a Lakemaster card (which I expect is more accurate data than the default map data).

- Water Levels - These were what I was reading for depths through the transducers.  I was able to see depths on three different transducers, so I do not believe I have a faulty 'ducer causing this oddity.

[John Jarosz]

Darn!!! I was hoping that it made sense to you & you would explain it to me.

Here's what I gathered from your post:

So, let's say your water level is reading 20', which means you're in 20 fow, (correct?)

The Lakemaster card shows that it should be 25'. Is that correct?

Answer those two question, & we'll start from there. The one thing that I'm thinking that kind of gums things up is that "where does the Lakemaster get its data?" If you look at an NOAA paper map (which us old-timers have), those depths come from the Low Water Data.

Thanks

[John Jarosz]

Joe...or is it the other way around? You're showing water level to be 20', i.e., you're in 20 fow

You Lakemaster graph shows 15' on the map?

thanks

[Joe Dolan]

What I am seeing is that the lakemaster maps are showing I should be in 25 fow but the transducers are showing my actual depth of 20 fow.  

I am used to seeing differences between the two, but seeing it in one area of the harbor and not in other areas (like the gap openings) is what confused me.  I would have expected more consistency.

[John Jarosz]

Were you seeing this difference in the Bflo Harbor channel & not in the gaps, i.e. were you like in the middle of the channel & away from the walls or gaps?

[Joe Dolan]

Correct.  Gaps, entries to channels (like the coke channel and the marina), and out in the lake (outside the walls) were matching up perfectly.  I was seeing this difference (5ft difference) in the middle of the harbor.

[John Pensyl]

If it isn't live mapped, its make believe.

[John Jarosz]

Here's my theory (theory only). 

When Andy Lacko & I were trying to retrieve one of the telemetry receivers near the Cargill, we noticed there was a WICKED CURRENT at the location. Andy mentioned something that it was that way because inside the Harbor is dredged, so the water running from the lake goes to fill the Harbor. 

The middle of the Harbor is on, I believe a three year dredging cycle by the Corps. Lakemaster maps & NOAA paper maps are not "live" & they get their reference depths. from the Low Water Datum. The map on your Lakemaster isn't updated every time you go out. There is a current out in the lake called the littoral drift, i.e. if you drop a bucket of sand at Sturgeon Point, that bucket will eventually move down the Niagara. 
When silt makes it to the South Gap, that current is ripping so it won't accumulate in the gaps & your depth finder will read the same (or close to) the map. 

Add a foot & a half to the Lakemaster reading & that should be your graph reading. When you get to the middle of the channel, the current slows & the silt falls down to the bottom. The Lakemaster reading isn't going to change because it's taken off NOAA readings on a map.

That's why the Corps has to dredge periodically.
Here's what you should do to verify:

1. Go out to the middle of the channel & spot-lock 

2. Drop an anchor (not heavy) straight down & measure the amount of line you have out

3. Compare your Lakemaster to your depth finder graph to the actual measurement of your anchor line. 

I bet the anchor line matches your Depth finder, which will be shorter than what your Lakemaster shows. 

and that, in 13,000 words or less is what you're seeing. If you did that right after the harbor is dredged, everything would match. The difference you're seeing is the depth of the silt plus the difference between the actual lake level & Low Water Datum (about a foot & a half)

WHEW!! I'll buy you a beer at the Christmas party for that

[Joe Dolan]

I like that!  Definitely logical as to why I was seeing it where I was seeing it and not elsewhere.  Thanks, John!