Lakes & reservoirs

Guide Curves and Rule Curves Explained

By the Water Level Atlas team · Last reviewed Oct 21, 2026

A guide curve (also called a rule curve or operating curve) is a reservoir's planned water level for each day of the year. It usually dips in winter to leave room for floods and rises in spring and summer to store water. Operators use it as a target, not a promise, so the real lake level often sits above or below the curve.

What is a guide curve?

A USGS paper on reservoir management describes rule curves as "seasonal targets for water levels" that balance competing uses such as flood control, irrigation, and fish and wildlife. On a graph, the curve is a line running across the calendar, with elevation on the side.

Operators use different names for the same idea, including guide curve, rule curve and operating curve. TVA also uses a flood guide for its tributary reservoirs: its Inspector General explains that any time a tributary lake rises above its flood guide, it is out of line with TVA's flood storage rules.

For each USACE project, the curve is part of a water control manual. A 2018 Federal Register notice from USACE describes these manuals as guiding reservoir management "during times of high water, low water, and normal conditions," and says they contain "drought plans and zones" that tell managers when to reduce or increase releases.

Why does the curve change through the year?

Because the risks change. In seasons with high flood risk, the curve is lower, so more flood storage is empty. In seasons when the lake needs to hold water for summer recreation, water supply or power, the curve is higher.

TVA's Office of the Inspector General reported that two USACE district offices said each of their projects has a guide curve, with water drawn down in winter for flood storage ahead of spring rains and raised in summer for recreation. See winter drawdown.

Not every lake follows that shape. Some have nearly flat curves, and a few hold more water in winter than summer.

What are action zones?

Some USACE lakes split the conservation pool (the water below full pool) into bands called action zones or drought zones. The USACE Mobile District defines action zones as "regions within the conservation storage where operational rules effect a temporary reduction in firm services."

At Allatoona Lake in Georgia, for example, the 2015 manual describes four zones:

Zone	What the manual says it indicates	Example of what changes
Zone 1	Normal to wetter than normal	Typically up to 4 hours of peak hydropower
Zone 2	Reduced service	Typically up to 3 hours of peak hydropower
Zone 3	Drier than normal or impending drought	Typically up to 2 hours; careful long-range planning
Zone 4	Severe drought	Peak generation typically suspended; minimum flow kept

The lower the lake falls through the zones, the more the operator holds back to protect the water that is left. The exact zones, and what changes in each, are different at every lake.

The USACE Savannah District uses a similar idea with numbered drought trigger levels. As the lakes fall through each level, its drought plan lowers the target release from Thurmond Dam, from 4,200 cfs (about 119 m³/s) down to as low as 3,100 cfs (about 88 m³/s) in winter at the most severe level.

Why is my lake above the guide curve?

Usually because of recent rain. When inflow is high, operators store water above the curve temporarily and release it at a rate the river below can safely carry. TVA's Inspector General noted weeks when heavy rain pushed some TVA reservoirs up to their flood guide level. Being above the curve after a storm is normal. See what is full pool.

Why is my lake below the guide curve?

Common reasons include:

Drought or a dry spring. Inflows were too low to follow the curve upward.
Downstream needs. Water had to be released for water supply, water quality, navigation or minimum flows.
Planned work at the dam or shoreline.

In drought, plans may also allow lakes to recover above the normal curve when rain returns. The Savannah District's 2024 briefing lists a plan "to allow pools to refill above guide curve up to summer full pool."

Can a guide curve be changed?

Yes, but it takes time. The USGS paper says changing rule curves at USACE reservoirs is complicated and involves many constraints and competing demands. Changes usually go through studies, public comment and an updated water control manual.

Using this on Water Level Atlas

Our lake pages do not draw the operator's guide curve unless we have it from the official source. Instead we show the normal range for the date, built from the middle 50% of this lake's own past years. That band often follows a similar seasonal shape. For the official curve and current zone, check the operator's own website. See who manages US reservoirs.