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. Digital images of these photographs are available on the Web site http://nh.water.usgs.gov
The U.S. Geological Survey (USGS) today issued preliminary estimates of the magnitudes of floods experienced throughout New Hampshire from May 13 through May 17. The highest ever flows recorded by the USGS occurred at 12 rivers in central and southern New Hampshire. They include the Lamprey River at Newmarket, Salmon Falls River at Milton, Cocheco River near Rochester, Exeter River near Brentwood, Soucook River near Concord, Warner River at Davisville, Piscataquog River near Goffstown, Beaver Brook at North Pelham, and the Spicket River near Methuen, Massachusetts.
"Flows during the flood peak for the Lamprey, Exeter, Warner, Soucook, Merrimack, and Spicket Rivers generally were at or exceeded those peaks that would be expected an average of once in a 100-year period, termed the 100-year return interval," says Kenneth Toppin, USGS hydrologist.
Examples of recorded peak flood flows highlight the magnitude of the flooding. The peak flood flow in the Lamprey River at Newmarket was 9,100 cubic feet per second on May 16; normal flow for this date is 366 cubic feet per second. The Warner River at Davisville had a peak flow of 9,300 cubic feet per second on May 15; normal flow for this date is 387 cubic feet per second. The Piscataquog River near Goffstown had a peak flow of 10,100 cubic feet per second on May 14; normal flow for this date is 426 cubic feet per second. The Merrimack River at Manchester peaked at 74,800 cubic feet per second; the 3rd highest flow since peaks in 1936 from spring runoff and in 1938 from a hurricane. The Spicket River near Methuen, Massachusetts, peaked at 2,080 cubic feet per second on May 16; the highest flow since streamflow monitoring began on this river in 2000.
"Many other rivers throughout southern part of the state had peak flows that exceeded the 5, 10 and 25-year return intervals," said Toppin.
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.