Is full pool the same as 100 percent full?

Yes, by definition. Percent of conservation pool is 100 at the top of the conservation pool. Readings above 100 mean water is in the flood pool.

Can a lake go above the flood pool?

Rarely. Above the flood pool is the surcharge zone, up to the top of the dam, used only in an extreme flood when the spillway is passing everything it can. Agencies treat any reading above the flood pool as an emergency condition.

Why do some lakes have no flood pool?

Because they were built for water supply, irrigation or power only, by a city, river authority or utility rather than the Corps. They fill to a spillway crest and anything above that simply flows over. Most non-Corps Texas lakes are like this.

What datum are the elevations in?

Each lake page states its datum, usually NGVD29 or NAVD88. The difference between the two is a few tenths to a couple of feet depending on location, so never compare a NAVD88 elevation to an NGVD29 pool table.

Full pool vs flood pool

Every lake page on this site draws the same four lines on the staff gauge: the top of the flood pool, full pool, the bottom of the conservation pool, and today's level. Those lines come from the operating agency's water control plan, and once you know what each one means the raw elevation stops being a mystery number.

Full pool: the top of the conservation pool

Full pool is the elevation the agency intends to hold under normal conditions. Formally it is the top of the conservation pool, sometimes called the top of the multipurpose pool or, on hydropower lakes, the top of the power pool. Water stored below this line is the reservoir's working supply: it is released through the year for municipal water, irrigation, navigation, hydropower and downstream flows, and it is the level around which docks, ramps and marinas are built. When someone says a lake is "full", this is the line they mean. The lake pages show every reading as feet above or below full pool for exactly that reason.

Flood pool: temporary storage above full

On a Corps of Engineers reservoir, the space between full pool and the top of the flood pool is kept empty on purpose. When a storm pushes inflow far above what the dam can safely pass downstream, the extra water is held in this zone, and the lake rises. Afterwards the agency releases the stored floodwater as fast as downstream channels allow, and the lake falls back to full pool. The flood pool can be enormous: Canyon Lake in Texas has 34 ft of it, Table Rock in Missouri 14 ft. A lake that reads 110 percent full is not "over-full" in any alarming sense, it is doing its job.

Lakes built purely for water supply or irrigation, which is most of the non-Corps reservoirs in Texas, have no flood pool. They fill to the spillway crest and excess water simply flows over. On those pages the flood line is absent and the gauge runs from the bottom of the conservation pool to full pool only.

Bottom of the conservation pool and the inactive pool

Below full pool, the conservation storage runs down to a bottom elevation. Below that is the inactive or dead pool: water that sits under the lowest outlet and cannot be released by gravity. The percent of pool figure on each lake page is the share of conservation storage remaining, measured by volume where the agency publishes it and by elevation as a fallback. Because a reservoir is a bowl, the top few feet hold far more water than the bottom few, so a lake 10 ft below full may still be 70 percent full.

Top of dam and surcharge

Above the flood pool is a final margin, the surcharge zone, up to the top of the dam. It exists to pass the design flood without overtopping and is never planned storage. A few lake pages list top of dam so you can see how much margin there is above the flood pool.

Reading the gauge

The orange marker is today's level. If it sits at the solid full pool line, the lake is at its normal level. Above it and below the dashed flood line, the lake is holding floodwater and will be lowered. Below full pool, the distance to the dotted bottom line shows how much of the working supply is left. Combine the marker with the 7-day and 30-day change on the same page: a level well below full and still falling in midsummer is a drought; the same level falling in October is usually a scheduled drawdown.

Datums: why two lakes cannot be compared by elevation

Elevations are measured above a national vertical datum, NGVD29 on most older Corps projects and NAVD88 on newer ones and in Texas. A lake's pool table and its gauge use the same datum, so the feet-from-full figure is always consistent within a page. Across pages, raw elevations mean nothing: a 500 ft lake in Georgia and a 6,000 ft lake in Colorado are both simply at their own full pool or not.