New Hampshire Flood Data Released by USGS

The U.S. Geological Survey (USGS) today issued preliminary estimates of the magnitudes of floods experienced on several rivers in southwestern New Hampshire on October 9 and 10. Flows on the Cold River at Drewsville, just downstream of Alstead, were estimated at 15,000 cubic feet per second based on high-water marks surveyed inside the discontinued USGS streamgage at the Route 123 bridge. Average flows at Drewsville are about 100 cubic feet per second. Cold River levels in the October 9 flood peaked at 23.7 feet at the streamgage. The previously recorded maximum level was 12.3 feet, at a flow of 6,700 cubic feet per second, in 1973. Flows during the flood peak greatly exceeded those that would be expected an average of once in a 100-year period, called a return interval. 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. USGS officials advised that the very large flows experienced on the Cold River may not be representative of natural conditions due to the sudden release of water impounded behind a blocked culvert that failed upstream in the basin.

Flows at the USGS streamgage on the Ashuelot River in Hinsdale were measured at 10,600 cubic feet per second, with a level of 9.4 feet, which also exceeded the 100-year return interval. However, larger floods have been experienced in the past, prior to the construction of the Surry Mountain and Otter Brook Reservoirs by the U.S. Army Corps of Engineers in the 1940s and 50s. The largest flood on record for the Ashuelot River at Hinsdale was 16,600 cubic feet per second at a level of 20.2 feet in 1936.

In Hillsboro, high water levels measured at the discontinued USGS streamgage on Beards Brook reached 9.9 feet, 3.4 feet above the previous record set in 1959. Discharge was approximately 4,500 cubic feet per second, with a return interval of approximately 80 years. Lower flood levels, which ranged in size from 10- to 20-year return intervals, were also recorded at USGS gages on the Sugar, Contoocook, and Warner rivers.

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.

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