Definitions

100-year Flood: The term "100-year flood" is often used as an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. While unlikely, it is possible to have two 100 or even 500-year floods within years or months of each other.

500-year Flood: The term “500-year flood” is often used as an attempt to simplify the definition of a flood that statistically has a .02-percent chance of occurring in any given year. While unlikely, it is possible to have two 100 or even 500-year floods within years or months of each other.

Flood Stage: The National Weather Service, based on the desires of the local community, establishes the "flood stage" gauge height for any given community. The flood stage gauge height is often the stage where damages begin to occur. Many communities desire to use the flood stage gauge height as an early warning alert, prior to the onset of significant damages. Significant damages may not occur until river levels are several feet above flood stage. Additionally, conditions along some rivers may have changed since the gauge and flood stages were established and reaching the flood stage may or may not result in actual damages. Again, stages are site-specific, so feet above flood stage at one location can’t be compared to another.

River Stage: A site-specific measurement of river-level referenced as the height in feet above a designated zero reference point, called the gauge zero, at the site. The zero reference point is sometimes, but not always, chosen as the elevation of the river bottom. Normally, stage values are always positive. Drought conditions could cause the river level to fall below gauge zero, and the stage reading at that time would be negative. Since each gauge was established independently at each location, the stage reading is good for that location only and cannot be compared to other locations. For example, a stage of 30 feet at Fargo cannot be compared to a stage of 30 feet at Grand Forks. The only way direct comparisons between two gauges can be made is by converting river stage to elevation by adding the stage to the gauge zero elevation.

Acre Foot: An acre-foot is defined as the volume of water needed to cover one acre in area (one acre is 208 feet by 208 feet) with one foot of water. This volume of water is equal to 325,851 gallons. It takes approximately two acre-feet of water to fill an Olympic-size swimming pool.

Cubic Feet Per Second: Cubic feet per second (cfs) is a unit of measurement commonly used to describe the rate of flow in streams and rivers. A cubic foot of water equals the amount of water that would fit into a 1-foot by 1-foot by 1-foot box or cube. This amount of water is equal to 7.48 gallons and weighs 62 pounds.