Flood pool, conservation pool and dead pool

Engineers divide a reservoir into zones stacked on top of each other. Knowing them makes water level numbers easier to read.

From bottom to top

• Dead pool: water below the lowest outlet. It cannot be released through the dam by gravity. When a lake approaches dead pool, as some Western reservoirs have in severe drought, the dam can no longer pass water downstream through its outlets.
• Inactive pool: a layer just above dead pool, often reserved for sediment and minimum hydropower operation.
• Conservation pool: the working range used for water supply, hydropower, navigation and recreation. Its top is usually what people call full pool.
• Flood pool: empty space above full pool kept to catch floodwater. After big storms the lake rises into it, then operators release the extra water over days or weeks to avoid flooding downstream.
• Surcharge: water above the top of the flood pool, reached only in extreme events, up to the spillway crest and beyond.

What it means for boaters

When a lake is in its flood pool, expect floating debris, flooded ramps and parking lots, submerged docks, closed campgrounds and strong currents near the dam. Many operators post no-wake rules to protect flooded shorelines.

A lake well into its conservation pool but below full pool is the normal working state for much of the year. The water level page for each lake shows whether today’s reading is high, low or near normal for the month.