A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Fault
The zone of displacement in rock formations resulting from forces of tension or compression in the Earth's crust. Faults can cause barriers or conduits to the sub-surface flow of water.

First Flush
The delivery to a stream or lake of a large load of pollutants during the early part of storms because of rapid runoff of accumulated pollutants.

Flood
An overflow of water onto lands that are used or usable by man and not normally covered by water. Floods have two essential characteristics: The inundation of land is temporary; and the land is adjacent to and inundated by overflow from a river, stream, lake, or ocean. See also Channel flow.

Flood Stage
The specific elevation at which floodwater begins to overflow the natural banks of a stream or body of water. For example, flood warning news reports may refer to a rising river being "within 1 foot of flood stage."

Flood, 100-year
A 100-year flood refers to a flood level with a 1 percent chance of being equaled or exceeded in any given year. It does not refer to a flood that occurs once every 100 years.

Floodplain
The generally flat area adjacent to rivers that is periodically flooded. Evolving over hundreds or thousands of years, the size of floodplains is related to the frequency of flooding, the energy of the flow of the river when in flood, and the amount of sediment in the river system. Most communities have zoning laws that restrict building development on flood plains.

Flowing Well
A well that taps ground water under pressure so that water flows over the well casing without pumping.

Fracture Zone
A zone of cracks or fissures within rocks. Individual fractures may be of limited extent but are often connected with others. Fractures can occur for many different geologic reasons.

Freshwater
Water that contains less than 1,000 milligrams per liter (mg/L) of dissolved solids; generally, more than 500 mg/L of dissolved solids is undesirable for drinking and many industrial uses. Sea water contains about 35,000 mg/l of dissolved solids, mostly sodium and chloride.

Frost Table
The surface within permafrost soils to which seasonal thawing extends. Depth is usually less than 3 feet. The insulation effect of vegetation influences thawing depth. If freezing "traps" liquid water below it may be squeezed to the surface to form a temporary "icing."