New Hampshire Flood Data for April 2007 Released by USGS

Note to Editors: Return intervals are used by hydrologists to describe the magnitude and frequency of floods and represent the average interval of time over which floods of similar magnitudes are expected to occur.

The U.S. Geological Survey (USGS) today issued preliminary estimates of the magnitudes of floods experienced throughout New Hampshire from April 16 through April 18. The highest ever flows recorded by the USGS occurred at 5 rivers in southern New Hampshire. They include the Oyster River near Durham, Cocheco River near Rochester, Isinglass River near Dover, Piscataquog River near Goffstown, and Contoocook River at Peterborough.

Flows during the flood peak for the Oyster, Lamprey, Exeter, Cocheco, Isinglass, Piscataquog, and Contoocook Rivers generally were at or exceeded those peaks that would have a 1 in 100 chance of being equaled or exceeded in any given year, termed the 100-year return interval, says Keith Robinson, USGS New Hampshire-Vermont Water Science Center Director.

Examples of recorded peak flood flows highlight the magnitude of the flooding at several long-term streamgages. The peak flood flow in the Lamprey River at Newmarket was 8,450 cubic feet per second on April 18; normal flow for this date is 654 cubic feet per second. The peak flood flow in the Oyster River near Durham was 1,300 cubic feet per second on April 17; normal flow for this date is 47 cubic feet per second. The Contoocook River at Peterborough had a peak flow of 4,150 cubic feet per second on April 16; normal flow for this date is 343 cubic feet per second. The Piscataquog River near Goffstown had a peak flow of 11,500 cubic feet per second on April 16; normal flow for this date is 920 cubic feet per second.

Many other rivers throughout southern part of the state had peak flows that exceeded the 5, 10 and 25-year return intervals, said Robinson.

USGS hydrologists from the New Hampshire-Vermont Water Science Center measured the flows and height (termed stage) of the flood waters in rivers statewide during the past week to be able to accurately define the peak flood flows. More detailed information on where elevated stream flows and height occurred during the past week can be found in the attached table and on the Web site http://nh.water.usgs.gov.

Graphs and tables showing the real-time streamflow data collected at USGS gages in the New Hampshire network for the last 31 days, and for the historical periods of record, can be found on the Web site http://nh.water.usgs.gov/WaterData/station_map.htm.