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Annual peak-flow frequency data from 231 U.S. Geological Survey streamflow-gaging stations in North Dakota and parts of Montana, South Dakota, and Minnesota, with 10 or more years of unregulated peak-flow record, were used to develop regional regression equations for exceedance probabilities of 0.5, 0.20, 0.10, 0.04, 0.02, 0.01, and 0.002 using generalized least-squares techniques. Updated peak-flow frequency estimates for 262 streamflow-gaging stations were developed using data through 2009 and log-Pearson Type III procedures outlined by the Hydrology Subcommittee of the Interagency Advisory Committee on Water Data. An average generalized skew coefficient was determined for three hydrologic zones in North Dakota. A StreamStats web application was developed to estimate basin characteristics for the regional regression equation analysis. Methods for estimating a weighted peak-flow frequency for gaged sites and ungaged sites are presented.

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DEFINITION OF TERMSClimatic year is the 12-month period April 1 through March 31. The climatic year is designated by the calendar year in which it begins.Cubic foot per second (ft3/s) is the rate of discharge representing a volume of 1 cubic foot passing a given point during 1 second and is equivalent to 7.48 gallons per second or 448.8 gallons per minute or 0.02832 cubic meters per second.Discharge is the volume of water in a stream at a given point and for a given period of time. Discharge is often used interchangeable with the term "streamflow".Drainage area is the area, measured in a horizontal plane, enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into the stream upstream from the station. Figures of drainage area given herein include all closed basins, or noncontributing areas, within the area unless otherwise specified.Drainage basin is a part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.Gage height is the water-surface elevation referred to some arbitrary gage datum. Gage height is often used interchangeable with the more general term "stage", although gage height is more appropriate when used with a reading on a gage.Gaging station is a particular site on a stream, canal, lake, or reservoir where systematic observations of hydrologic data are obtained.Ground water is the water in the ground that is in the zone of saturation, from which wells, springs, and ground-water runoff are supplied.Hvdrologic unit is a geographic area representing part or all of a surface drainage basin or distinct hydrologic feature as delineated by the Office of Water Data Coordination on the State Hydrologic Unit Maps; each hydrologic unit is identified by an eight-digit number.Instantaneous discharge is the discharge at a particular instant of time. Mean is the arithmetic average of a list of values.Mean discharge is the arithmetic mean of individual discharges during a specific period. Period of record is the time during which a stream-gaging station is in operation and for which the records are published.Probability of occurrence is the likelihood that an event will occur. Probabilities are generally expressed as a decimal number between 0 and 1. If the probability is 0, the event will not occur, if the probability is 1, the event will occur absolutely. Probability also can be expressed as a percent, where 0 percent corresponds to 0 probability and 100 percent corresponds to a probability of 1.Recurrence interval is the average time interval between occurrences of a hydrologic event of a given or greater magnitude, usually expressed in years.Regulation is the artificial manipulation of the flow of a stream. Stage see "gage height"Standard deviation is a measure of the variability of the values in a list of values.Stream-gaging station is a particular site on a stream where a record of disch...