Low-flow frequency curve.--A graph showing the relation between the magnitude and frequency of the annual lowest mean flow for a given number of consecutive days. Low-flow frequency is expressed as the average interval, in years, between recurrence of an annual minimum flow less than that shown by the magnitude scale. The low-flow frequency relation is commonly presented as a family of curves with a separate curve for each number of consecutive days.Partial-record site.--A location on a stream, canal, lake or reservoir where limited streamflow data, usually consisting of streamflow measurements, are collected over a period of years for use in hydrologic analysis.Regulation. The artificial manipulation of the flow of a stream, such as, by the operation of reservoirs, cranberry bogs, or artificial lakes.Recurrence interval. The time interval, in years, within which a given hydrologic event will, on the average, be less or more than a given value one time. It is the reciprocal of the probability that the given hydrologic event will not be exceeded or will be exceeded in any one year. For example, a 20-year low-flow is one that has a 5 percent chance of not being exceeded in any one year.Water year. The 12-month period, October 1 through September 30, designated by the calendar year in which it ends.
From September 2004 to June 2006, the Delaware River in New Jersey, New York, and Pennsylvania experienced three major floods that caused extensive damage. The Federal Emergency Management Agency (FEMA) needed updated information on the flood magnitude and frequency for the eight active streamflow-gaging stations along the main stem Delaware River in New Jersey, New York, and Pennsylvania that included the three recent floods in order to update its flood insurance studies. Therefore, the U.S. Geological Survey (USGS) computed updated flood magnitude and frequency values following the guidelines published by the Interagency Advisory Committee on Water Data in its Bulletin 17B. The updated flood-frequency values indicate that the recurrence interval of the September 2004 flood ranged from 20 to 35 years, the recurrence interval of the April 2005 flood ranged from 40 to 70 years, and the recurrence interval of the June 2006 flood ranged from 70 to greater than 100 years. Examination of trends in flood discharges indicate no statistically significant trends in peak flows during the period of record for any of the eight streamflow-gaging stations.
Streamflow statistics were computed for 111 continuousrecord streamflow-gaging stations with 20 or more years of continuous record and for 500 low-flow partial-record stations, including 66 gaging stations with less than 20 years of continuous record. Daily mean streamflow data from water year 1897 through water year 2001 were used for the computations at the gaging stations. (The water year is the 12-month period, October 1 through September 30, designated by the calendar year in which it ends). The characteristics presented for the long-term continuous-record stations are daily streamflow, harmonic mean flow, flow frequency, daily flow durations, trend analysis, and streamflow variability. Low-flow statistics for gaging stations with less than 20 years of record and for partial-record stations were estimated by correlating base-flow measurements with daily mean flows at long-term (more than 20 years) continuous-record stations. Instantaneous streamflow measurements through water year 2003 were used to estimate low-flow statistics at the partial-record stations. The characteristics presented for partial-record stations are mean annual flow; harmonic mean flow; and annual and winter low-flow frequency. The annual 1-, 7-, and 30-day low-and high-flow data sets were tested for trends. The results of trend tests for high flows indicate relations between upward trends for high flows and stream regulation, and high flows and development in the basin. The relation between development and low-flow trends does not appear to be as strong as for development and highflow trends. Monthly, seasonal, and annual precipitation data for selected long-term meteorological stations also were tested for trends to analyze the effects of climate. A significant upward trend in precipitation in northern New Jersey, Climate Division 1 was identified. For Climate Division 2, no general increase in average precipitation was observed. Trend test results indicate that high flows at undeveloped, unregulated sites have not been affected by the increase in average precipitation. The ratio of instantaneous peak flow to 3-day mean flow, ratios of flow duration, ratios of high-flow/low-flow frequency, and coefficient of variation were used to define streamflow variability. Streamflow variability was significantly greater among the group of gaging stations located outside the Coastal Plain than among the group of gaging stations located in the Coastal Plain.
A network of 93 gaging stations that provide surface-water stage, flow (discharge), and tide-level data on a "realtime" basis through satellite, radio, and telephone telemetry is operating (May 2003) in New Jersey through a cooperative effort of the U.S. Geological Survey (USGS) and other agencies. The stream data from these stations are transmitted every 1 to 4 hours and then are immediately posted for viewing on the Internet. This fact sheet describes the "real-time" monitoring network, and the equipment used to measure stage and flow and to transmit the data for viewing on the Internet. Instructions for viewing the data are included. The agencies cooperating in the operation and maintenance of the "realtime" surface-water data network are Background This statewide network consists of several sub-networks that were created to provide time-critical surfacewater data as well as information on long-term hydrologic conditions and trends in stream stage, flow (discharge), and tide levels within the State of New Jersey and to make the data available quickly. The gaging stations in these "real-time" networks, located throughout New Jersey (fig. 1), provide time-critical information for the monitoring of floods, droughts, and daily streamflow conditions needed for public safety; water-supply management; and the daily operations of water supply and receiving-water discharges. This combined statewide network, for example, provides data that allow for timely flood warnings to the public and evacuations in flood-prone areas. The system provides upto-date observations of drought conditions for the optimum management of water supplies and up-to-date information on streamflow conditions for fishermen, canoeists, kayakers, boaters, and other recreational users. In addition, the data from this network can be used to estimate the most recent stream and tide conditions at nearby stations that do not have satellite telemetry. Surface-water stage and streamflow information typically is used by engineers, planners, water-supply managers, emergency-management personnel, and the general public for a variety of purposes. Some of the uses for this streamflow information include incorporation into the design of bridges, dams, flood detention and control struc-U.
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