Lakes & reservoirs
Why Do Lake Levels Change? Inflow, Outflow and More
By the Water Level Atlas team · Last reviewed Oct 21, 2026
A lake's level changes when the water coming in does not match the water going out. Rain, runoff and rivers add water. Dam releases, water withdrawals, evaporation and seepage take it away. On a reservoir, people also decide how much to release, so operating rules matter as much as the weather.
What is a lake's water budget?
Think of a lake like a bank account:
- Water in: rivers and streams, runoff from the land around the lake, rain falling on the lake surface, and in some places groundwater seeping in.
- Water out: flow out through the dam or outlet river, water pumped out for cities, farms or industry, evaporation from the surface, and seepage into the ground.
If more comes in than goes out, the level rises. If more goes out, it falls. The USGS puts it simply: a lake exists because water enters it faster than it can escape.
How much does rain raise a lake?
It depends on how big the watershed is compared with the lake. A large watershed draining into a small lake can raise it quickly.
The USACE Savannah District gives real figures for its three reservoirs. One inch of runoff from the local drainage area raises:
| Lake | Local drainage area | Rise from 1 in of runoff |
|---|---|---|
| Hartwell | 1,294 sq mi | about 1.2 ft (at 660 ft) |
| Richard B. Russell | 802 sq mi | about 1.5 ft (at 475 ft) |
| J. Strom Thurmond | 2,890 sq mi | about 2.2 ft (at 330 ft) |
Note that this is one inch of runoff, the water that actually reaches streams, not one inch of rain. Much rain soaks into the ground first, especially after dry weather.
What are reservoir releases for?
On a dammed reservoir, operators control most of the outflow through gates, turbines and spillways. Releases serve several purposes, which differ by project. The Savannah District lists flood risk management, hydropower, navigation, water supply, water quality, fish and wildlife, and recreation for its lakes. In practice, releases may be made to:
- Generate hydropower, often during hours of high electricity demand.
- Support navigation by keeping enough water in the river channel downstream.
- Supply water to cities, industry and irrigation, either from the lake or further downstream.
- Meet downstream flow requirements for water quality and fish. The Leesville Lake manual, for example, includes maintaining a continuous minimum flow in the creek below the dam.
- Empty flood storage after a storm, at a rate the river below can carry.
The Corps says each lake follows its own water control plan, and there is no single release formula for every lake. See guide curves explained.
Why doesn't the lake drop as soon as the rain stops?
During a flood, the dam holds water back on purpose. The Fort Worth District explains that releasing too much water too fast can make flooding worse downstream, while holding too much makes the lake rise higher. Operators balance the two using the plan, forecasts and downstream gauges. So a lake can keep rising after the rain ends, as runoff keeps arriving, and then fall slowly over days or weeks. See what is full pool.
How much does evaporation matter?
It varies a lot by climate, lake size and season. In its Leesville Lake manual, the USACE Huntington District says that for most reservoirs in that Ohio river basin the amount lost to evaporation is quite small. It gives average annual lake evaporation there of about 32 inches, but notes that the extra loss caused by the lake, compared with the land before it was flooded, is only about seven or eight inches a year.
How are natural lakes different from reservoirs?
A natural lake forms where water collects in a low spot in the land. Its outflow is set mostly by the shape of its outlet, so the level follows the weather: up after wet spells, down in droughts.
A reservoir is a lake created by building a dam on a river, as the USGS notes. Many are still called "lakes". Their level reflects weather plus human decisions: seasonal targets, power demand, water supply contracts and downstream needs. Some natural lakes also have small control dams at the outlet, so they behave partly like reservoirs.
Why is my lake low even though it rained?
Common reasons, all of which depend on the specific lake:
- Dry ground soaked up most of the rain, so little ran off.
- The rain fell below the dam or outside the watershed.
- The lake is following a seasonal plan, such as a winter drawdown.
- Releases are being made for downstream needs.
Check the operator's own updates for the reason at your lake.
Using this on Water Level Atlas
Lake pages show the current level and its change over 24 hours, 7 days and 30 days, so you can see how fast it is moving. Where the operator publishes them, we also show inflow and outflow, which tell you whether the lake is filling or emptying. The normal range for the date shows whether today's level is typical for the season. Our methodology page explains the data sources.