How we measure

The reading

The current level is the latest value the agency publishes for the lake's pool elevation gauge, in feet above sea level on the vertical datum the agency uses (usually NGVD 1929; each page names it). Corps of Engineers gauges report every 15 minutes to an hour. Reclamation and USGS publish one value per day, so those pages show the day's reading. For history we average each day's readings into one daily value in the lake's local time.

Before any calculation, readings that jump more than 10 feet away from the surrounding two weeks are treated as telemetry glitches and set aside.

Full pool

For Corps lakes, full pool is the top of the conservation pool (or the district's equivalent: top of normal pool, top of power pool, or the guide curve) as published in the Corps water management database. Many Corps lakes lower it in winter; we compare against the value that applies on the day of the reading, and say so when it changes during the year. For Reclamation reservoirs we use the full pool elevation Reclamation publishes for the dam. Natural lakes, such as Great Salt Lake and Lake Champlain, have no full pool, so their pages compare only with the normal for the date.

Percent of conservation storage

Where the Corps publishes current storage and the storage at the top and bottom of the conservation pool, we report the water above the bottom as a share of the space between them. We only show it when the storage reading is from the same few days as the elevation and agrees with it (below full pool means under 100%). Otherwise the page shows feet only.

Normal for the date

For each past year we take the average level over the seven days centred on today's calendar date. With at least five past years, the median of those yearly values is the normal, and the 10th to 90th percentile is the normal range drawn on the chart. We use up to the 30 most recent years. The percentile is the share of past years that were lower than today on the same date.

• Below the 10th percentile: much lower than usual.
• 10th to 25th: lower than usual.
• 25th to 75th: about normal.
• 75th to 90th: higher than usual.
• Above the 90th: much higher than usual.

Last year and trends

“A year ago” is the daily value 365 days before the reading (or the nearest day within three). The 7-day and 30-day changes compare the reading with the daily value that many days earlier.

Records

Highest and lowest levels are the extremes in the daily record we hold, which starts in the year shown on each page. Agencies' official records may reach further back.

The dock check

For each month we keep the spread of daily levels over the last ten years. The tool reads off how many of those days were at or above the elevation you type.

When a page is held back

A lake page is kept out of search results, and left out of the sitemap, when its latest reading is more than 10 days old, when it has fewer than five past years on today's date, or when a reservoir has no published full pool. The page still works for visitors and says why.

Updates

The site is rebuilt from fresh readings several times a day. Agency data are provisional and can be revised; we pick up revisions on the next rebuild.