Mount St. Helens, Washington
Summary of the Floods of November - December 1995

The 1996 water year realized many periods of high water and flooding throughout Washington in a year of above normal precipitation. Baring, on the west slope of the Cascade Range, received over 150 inches of precipitation during the year, comparable to wetter places on the west side of the Olympic Mountains (National Oceanic and Atmospheric Administration, 1995). The wettest month was November, with some areas along the west slope of the Olympic Mountains and Cascade Range receiving over 30 inches of rain -- Baring received almost 41 inches (National Oceanic and Atmospheric Administration, 1995). November, along with February, also experienced the worst flooding in the State. At some stations, November produced the highest peak, but for most, it was in February. The combination of heavy rainfall and high snowmelt in February produced record-setting peak discharges at many stations in the southern half of the State. The driest months were March, June, and July.

Streamflows during November were above average throughout the State except for the northeast quarter, where they were average. Flows approached or exceeded peaks of record for some stations due to very heavy rainfall from three series of storms (November 5-8, 10-11, and 27-30). Several locations on the west side of the Olympic Mountains and Cascade Range received over 30 inches of rain during the month. About 10 inches of rain were recorded in the Puget Sound region. Peak streamflows on November 29-30 approached peaks of record in the Snoqualmie, Sauk, upper Nooksack, and Cowlitz River Basins and exceeded instantaneous peaks of record at some stations in the Skagit, lower Nooksack, and Wenatchee River Basins.

In December and January, rains continued, but precipitation averaged only slightly above normal in the State. Several stations on the west side of the Olympics received over 20 inches of rainfall in December and again in January. Streamflows averaged above the mean throughout the State in December and ranged from normal to above normal in January. Streamflow at several stations in small basins approached or exceeded instantaneous peaks of record in December -- Canyon Creek near Cedar Falls (station 12116100, December 4, 107 ft3/s, peak of record is 131 ft3/s), Boxley Creek near Edgewick (station 12143900, December 3, 260 ft3/s, new record), and Duckabush River near Brinnon (station 12054000, December 12, 9,240 ft3/s, new record). Arctic air moved into the State on January 27, producing some snow on January 27-29, but brought clearer skies and extremely cold temperatures afterward. The air temperature at Davenport, near Spokane reached -28 degrees Fahrenheight (degrees F) on January 31 (National Oceanic and Atmospheric Administration, 1996). Hydrographs for all stations in the State dropped during this period as freezing weather prevented snowmelt and, along with drier weather, reduced streamflows.

The cold weather continued into early February, but was soon replaced by a warmer, very wet series of storms February 5-9 that produced heavy rain, high rates of snowmelt, and record flooding throughout much of the State. Streamflows were above normal throughout Washington except for the northwest corner. Very heavy rains on February 5-9 -- Cougar, 18 inches; Packwood, 12 inches; Baring 15 inches; SeaTac, 6 inches; Plain, 6 inches (National Oceanic and Atmospheric Administration, 1996) on already saturated ground in western Washington, or on frozen ground in eastern Washington accompanied by snowmelt induced by the rain and warmer air (freezing levels above 6,000 feet) produced high flows at most stations in the State. On February 8-9, flows at many stations in the southern half of the State exceeded the peak flow of record. At a number of stations, such as in the Cowlitz Basin, peak February flows were just slightly higher than the near-record peak flows in late November. At many gaging stations in western Washington, the February flooding was the worst since November, 1990, when some stations in the Skagit and Nooksack River Basins in northern Washington recorded their peak discharges of record. At many eastern Washington stations, peak flows were below the peak of record, such as at Tucannon River near Starbuck (station 13344500, 5,580 ft3/s, on February 9, peak of record 7,980 ft3/s on December 22, 1964). At some stations where temperatures remained near freezing, only a little rise in streamflow was noted on February 8-9 (Methow River near Pateros).

Cowlitz River at Castle Rock

[Graphic,13K,GIF]
Flood of November-December 1995
-- stage adjusted to feet above sea level (NAVD88)

[Graphic,11K,GIF]
Flood of February 1996
-- stage adjusted to feet above sea level (NAVD88)

[Graphic,13K,GIF]
Flood of November-December 1995
-- stage in feet above gage

[Graphic,12K,GIF]
Flood of February 1996
-- stage in feet above gage

From: U.S. Geological Survey, Water Resources Division, Cascades Volcano Observatory, 1996
SUMMARY: Late November high water:

Heavy rain and warm temperatures in late November resulted in the highest runoff in years for the Toutle and Cowlitz systems. Members of the hydrologic surveillance section, with assistance from others in the office, made numerous discharge measurements before, during, and after the peak flow to define changes in stage/discharge ratings. Suspended sediment samples were collected at three sites -- Muddy River, South Fork Toutle River at Camp 12, and Toutle River at Tower Road. Some significant preliminary specifics were:

• Highest water surface elevation yet seen at the reservoir behind the Corps of Engineers' Sediment Retention Structure. Reservoir elevation reached a maximum of 943.25 ft on Nov. 30. For the first time since its activation in 1987, water flowed through the overflow spillway.

• Highest stage and highest discharge measurement made at the North Fork Toutle River below the Sediment Retention Structure. Peak flow of 6,020 cubic feet per second (cfs) occurred at 0045 on Nov. 30 at peak stage of 59.56 ft. Discharge was measured at 5,170 cfs at 1238 on Nov. 30, at a stage of 59.10 ft.

• Highest stage and discharge at Toutle River at Tower Road since April 1991. Peak stage was 16.51 ft at 0045 on Nov. 30, with a computed discharge of 23,400 cfs. Discharge measurements were made on Nov. 28 and Nov. 30, of 19,100 cfs and 17,500 cfs, respectively.

• Highest stage and discharge at Cowlitz River at Castle Rock since the period of record flood in 1933. Computed peak flow of 105,000 cfs occurred at 0345 on Nov. 30, at a stage of 31.12 ft. This exceeded the 100-year flood at that station. A flow of 104,000 cfs was measured on Nov. 29 at 1602. (1933 peak stage was 31.6 ft, and peak flow was given as 139,000 cfs.)

Summary of the sediment data from the peak flow in late November is not available at this time due to the recent government shutdown and furlough. Samples have not yet been analyzed.

Please note that the stage and streamflow numbers given above are preliminary and subject to change as the records are worked and checked.