When a flood control system provides 100-year flood protection, it means that in any given year there is a one-in-100 chance that a storm might occur that is beyond the containment capacity of levees and reservoirs. Similarly, 200-year flood protection means there is a one-in-200 chance that a storm might occur that the system could not handle, and 500-year protection means there is a one-in-500 chance that a storm will overwhelm the system.

The graphic above shows the Sacramento area's lower estimated flood protection level as compared to other major river cities in the U.S. (source: Sacramento Area Flood Control Agency).

(For more information on Levee Construction and Flood History, please click the links at upper right.)