USGS vs. LPWD Lake levels?

Rainbowbridge

Rainbowbridge

Keeper of San Juan Secrets
apologies if this has been asked and answered......me memory ain't as good as it once was....now where was I? :unsure:

Oh yeah......USGS says today, 4-10-26 4AM level is @ 3530.6 .....https://waterdata.usgs.gov/monitoring-location/USGS-09379900/#dataTypeId=continuous-62615-0&period=P7D&showMedian=true&showFieldMeasurements=true

but LPWD says 3526.85 this morning....https://lakepowell.water-data.com/

and astute wordlings have noticed & mentioned the changes in the LPWD numbers of the last few days....(y)

So...why the appx 4 foot discrepancy? And wasn't there another site that listed levels mentioned some time back? :unsure:

As always...our WW is THE place for LP news & education....(y)
 
Rainbowbridge said:
apologies if this has been asked and answered......me memory ain't as good as it once was....now where was I? :unsure:

Oh yeah......USGS says today, 4-10-26 4AM level is @ 3530.6 .....https://waterdata.usgs.gov/monitoring-location/USGS-09379900/#dataTypeId=continuous-62615-0&period=P7D&showMedian=true&showFieldMeasurements=true

but LPWD says 3526.85 this morning....https://lakepowell.water-data.com/

and astute wordlings have noticed & mentioned the changes in the LPWD numbers of the last few days....(y)

So...why the appx 4 foot discrepancy? And wasn't there another site that listed levels mentioned some time back? :unsure:

As always...our WW is THE place for LP news & education....(y)
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I'm pretty sure I checked yesterday and the latest elevation showing was 3528. I wonder if there was a data correction or if there's an error somewhere.
 
LPWD gets its data from here:


That data set doesn’t show the bump a few days ago that LPWD does. So seems like something got out of sync. Perhaps LPWD is still showing some old provisional data that was later changed on the BoR site?

The inflows are calculated from the measured elevation changes and measured outflows. Small errors in elevation thus make big errors in inflow. So daily inflow numbers should always be taken with a big grain of salt on both LPWD and BoR. But if you average them over a handful of days they are accurate overall.

The CBRFC Lake Powell inflow is a better way to see inflow changes on day to day timescales:


But don’t expect that to line up perfectly with LPWD/BoR since they are entirely different methodologies. But the CBRFC will better tell you whether flows are trending up or down from day to day.
 
BartsPlace said:
Likely due to the fact that there are two different measuring systems. You can pick either one on the USGS site. They are approximately 3 feet apart.
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Not barking at the messenger. I can see inflow data being variable, depending on gauge site and method. But lake level is what it is wherever its measured. I dont understand the sudden change in the daily history. There were some green days that disappeared
 
upthecreek said:
Not barking at the messenger. I can see inflow data being variable, depending on gauge site and method. But lake level is what it is wherever its measured. I dont understand the sudden change in the daily history. There were some green days that disappeared
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The USGS data at least is marked provisional for a number of days. Don’t know the story for BoR, but in general measurements can have errors and then be revised later.
 
DVexile said:
but in general measurements can have errors and then be revised later.
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I would have presumed that the lake level, whether as an elevation, or feet of depth at the dam, would be a definable, repeatable number. Not subject to interpretation or revision like the annual jobs report. I guess it would be good news if they recalculate tomorrow and call it 3530' and we get a couple more weeks.
 
upthecreek said:
I would have presumed that the lake level, whether as an elevation, or feet of depth at the dam, would be a definable, repeatable number. Not subject to interpretation or revision like the annual jobs report. I guess it would be good news if they recalculate tomorrow and call it 3530' and we get a couple more weeks.
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Automatically and human collected data is typically provisional for a reason and there is rarely much rush in reviewing and revising it to an official number unless it is part of some time critical system. Most of the time no changes are necessary, but anything stuck in a lake is subject to errors from stuff growing on it, adhering to it, getting jammed in it, or a hundred other things. Which is why the data gets reviewed.
 
Worth noting at this point USGS data shows the lake level flat from March 31st through April 10th while BoR data shows it drop 1.14 feet from the 31st through the 9th (data for the 10th not posted yet).

The CBRFC simulated inflow (simulated because it’s based on combined measurements upstream from the lake rather than single direct measurement) implies there should be a flat period like the USGS data shows.

The USGS data is still marked provisional from March 24th onward. I’ve not found any way to tell when the BoR data is considered “official”.

So at this point it appears even the updated BoR elevations are likely still not quite right.

Sadly, if the CBRFC upstream flow measurements are correct, the flat spot is ending about now and in a few days inflow will be down around 5000 to 6000 cfs and the previous drop rate will resume.
 
upthecreek said:
I would have presumed that the lake level, whether as an elevation, or feet of depth at the dam, would be a definable, repeatable number. Not subject to interpretation or revision like the annual jobs report. I guess it would be good news if they recalculate tomorrow and call it 3530' and we get a couple more weeks.
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The "official" full pool level is 3,700' based on topographical data when they built the dam and spillways. Sorry that I don't have a good reference for this, but apparently they have more accurate surveys available since that time.

(After a little Google searching...)

I think that the 3,700' number is based on NGVD 1929, but there is NAVD1988 which could give you a water elevation a couple of feet higher. If you go to the site USGS Current Conditions for USGS 09379900 LAKE POWELL AT GLEN CANYON DAM, AZ you can select either the NAVD 1988 number or the NGVD 1929 number.

I just checked both and got the following numbers (4/11/2026 8:45 MST):

3,530.5' NAVD 1988
3527.6' NGVD 1929

(lakepowell.water-data.com says 3526.82 on 4/10/2026, which matches https://www.usbr.gov/uc/water/hydrodata/reservoir_data/919/csv/49.csv)

I think that most of the ramp cutoff numbers are based on the NGVD 1929 numbers. I have pictures from 4/6/2022 of the Bullfrog north ramp concrete forms and rebar at the water's edge with official lake level at 3,522.98' (lakepowell.water-data.com numbers).

I presume that they didn't extend the ramps further in 2023 because they had a massive snowpack in the upper basin and were expecting 30 - 40' of rise.

The bottom line is that there are enough people on this forum using the ramps that we should be able to get reports at least weekly on how deep the water is at the end of the concrete (or boiler plates) at each of the two available ramps.
 
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