Field Guide
Where Lake Powell's Water Data Comes From
Half a dozen sites publish a Lake Powell number and they do not always agree. Here is who measures what, which figures are forecasts rather than readings, and which source answers your question.
Search for Lake Powell's water level and you will get a government page, two or three tracker sites, a chart with a twenty-year line on it, and a number in a news headline. They will not all say the same thing, and the differences are not errors. They are measuring different quantities, on different days, and some of them are not measurements at all.
This is a guide to which source answers which question.
The measurement everyone is quoting
The Bureau of Reclamation operates Glen Canyon Dam and is the origin of nearly every Lake Powell figure in circulation. Its daily report for the reservoir carries the numbers that matter:
- Elevation — the lake surface in feet above sea level. This is the headline number, and it is the one described in how to read Lake Powell's water level.
- Storage — the volume of water held, in acre-feet. An acre-foot is the amount that would cover an acre a foot deep, roughly 326,000 gallons. Lake Powell holds a little over 24 million acre-feet at full pool.
- Percent of capacity — storage as a fraction of that full-pool volume. This is the figure that makes headlines, and it is the most misleading of the three, for a reason worth understanding.
- Inflow and release — how much water is arriving from the Colorado River and how much is being sent downstream to Lake Mead, usually in cubic feet per second or in acre-feet per day.
Why percent-of-capacity misleads. A reservoir in a canyon is not a bathtub. The lake is much wider near the top than near the bottom, so the relationship between elevation and volume is not linear. Dropping from full pool to half capacity costs far fewer vertical feet than dropping from half capacity to a quarter. A lake at "twenty-five percent" is nothing like a quarter of the way up the cliff, and people who convert one to the other in their heads always guess wrong. If you care about ramps and side canyons, use the elevation. If you care about the state of the Colorado River system, use the storage.
Who reads the gauges
The U.S. Geological Survey operates the stream gauges on the rivers feeding the reservoir and on the Colorado below the dam. If you want to know what is arriving rather than what is stored — the Colorado at Lees Ferry below the dam, the San Juan near Bluff, the Colorado near Cisco upstream — USGS is the source, and its data is available as near-real-time series.
This matters more than it sounds. Inflow in spring tells you what the summer will look like on the lake long before the reservoir surface reflects it.
The Natural Resources Conservation Service runs the snow monitoring network across the Rockies. Snowpack in the upper basin in February and March is the leading indicator for Lake Powell in June and July. When you read a confident prediction in spring about where the lake will peak, it came from here.
Forecasts are not readings
The single most common misreading is treating a projection as a measurement.
The Colorado Basin River Forecast Center publishes projected inflows. Reclamation's 24-Month Study, issued monthly, projects reservoir elevations up to two years ahead under a set of hydrology assumptions, and usually presents several scenarios — a most-probable case bracketed by wetter and drier ones.
These are genuinely useful for planning a trip several months out. They are not facts about the lake. A projected elevation for next August is a statement about a model; the number on Reclamation's daily page is a statement about the water. When a headline says the lake "will fall to" some figure, it is quoting the former, and the range around it is usually wider than the article implies.
The third-party trackers
Several independent sites republish Reclamation and USGS data with far better charts than the agencies produce, and they are genuinely useful — long historical series, elevation plotted against previous years, and the arithmetic already done for you. The Lake Powell water database sites are the best known of these, and they rank well because they answer the question faster than the official pages do.
Two cautions. First, they are republishers: the underlying reading still came from Reclamation, so a discrepancy between a tracker and the official page is a timing difference, not a second opinion. Second, none of them speak for the Park Service. A tracker can tell you the lake is at a given elevation; it cannot tell you whether the ramp you intend to use is open at that elevation, because that is an operational decision rather than a hydrological one.
Which source for which question
| Your question | Where to look |
|---|---|
| What is the lake at today? | Reclamation's daily reservoir report |
| Is it rising or falling right now? | Reclamation daily, or a tracker's recent chart |
| How does that compare with last year? | A third-party tracker's historical chart |
| What is arriving from the rivers? | USGS stream gauges |
| What will it do this summer? | NRCS snowpack, then Reclamation's 24-Month Study |
| Is my ramp open? | The National Park Service, and only the Park Service |
| Is there fuel at that dock? | The marina concessioner, by phone |
The last two rows are the ones people skip, and they are the ones that end trips. Hydrology tells you what the water is doing. It does not tell you what is open.
Reading a chart without fooling yourself
Mind the axis. A chart of the last five years and a chart of the last fifty tell very different stories about the same reservoir, and the short one usually looks more dramatic. Elevation charts that do not start at zero — most of them, sensibly — exaggerate the visual slope.
Mind the season. Comparing an October reading to a June reading tells you almost nothing, because the lake falls through autumn in normal years. Compare like months, or compare to the same date in previous years, which is exactly what the tracker charts are good at.
Mind the update time. The daily figure is a reading taken at a particular moment, and the sources refresh at different times of day. A few tenths of a foot of disagreement between two pages is not a controversy.
What to settle next
Once you know what the lake is doing, the practical question is what it changes — which ramps reach water, how far the walk to Rainbow Bridge has become, and whether the side canyons are inlets or beaches this year. That is covered in Lake Powell's water level, and what the number actually means.
If you are choosing where to get on the water, the level affects each access point differently, and the upper end of the lake feels a drop first — see which Lake Powell marina to launch from.