Streams in the study area drain relatively small basins; only three streams have drainage areas greater than 20.0 square miles (51.8 square kilometers), and only nine other streams have drainage areas greater than 10.0 square miles (25.9 square kilometers). Mean annual precipitation during the period 1931-60 ranged from about 25 inches (640 millimeters) near Hansville to about 70 inches (1,780 millimeter^) near Tahuya; it may be greater in the Green and Gold Mountain area. July is normally the month of least precipitation, and the lowest streamflows generally occur in September. Low-flow frequency data are tabulated for 90 streamflow sites in the study area; also listed are data for 56 additional sites which have insufficient measurements for frequency analysis but which have been observed having no flow at least once during the low-flow period. At the time of collection of field data, no effort was made to evaluate the effects of man on the streams' low flows. The eight streamflow gaging stations used in the low-flow analysis were all established after 1945, but all have had 10 or more years of record since then, and all were operated through the period of deficient rainfall, 1962-67. Low flows varied considerably during the periods of record of the stations. For example, variations in the 7-day low flows of Gold Creek near Bremerton ranged from 0.24 cubic foot per second (0.0068 cubic meter per second) in 1967 (at end of 6-year period of deficient rainfall) to 0.77 cubic foot per second (0.022 cubic meter per second) in 1948, a year of above average rainfall. Low-flow-frequency curves plotted from records of streamflow at the eight long-term gaging stations were used to determine data for low-flow durations of 7, 30, 60, 90, and 183 days. Regression techniques then were used to estimate low flows with frequencies up to 20 years for stations with less than 10 years of record and for miscellaneous sites where discharge measurements have been made.

Relations are provided to estimate the magnitude and frequency of floods on Washington streams. Annual-peak-flow data from stream gaging stations on unregulated streams having 10 years or more of record were used to determine a log-Pearson Type III frequency curve for each station. Flood magnitudes having recurrence intervals of 2, 5, 10, 25, 50, and 100 years were then related to physical and climatic indices of the drainage basins by multiple-regression analysis using the Biomedical Computer Program BMDO2R. These regression relations are useful for estimating flood magnitudes of the specified recurrence intervals at ungaged or short-record sites.Separate sets of regression equations were defined for western and eastern parts of the State, and the State was further subdivided into 12 regions in which the annual floods exhibit similar flood characteristics. Peak flows are related most significantly in western Washington to drainage-area size and mean annual precipitation. In eastern Washington they are related most significantly to drainage-area size, mean annual precipitation, and percentage of forast cover. Standard errors of estimate of the estimating relations range from 25 to 129 percent, and the smallest errors are generally associated with the more humid regions.

METRIC CONVERSION FACTORSMultiply foot (ft) cubic yard (yd 3 ) mile (mi) square mile (mi 2 ) cubic foot per second (fe /sec) By 0.3048 0.7646 1.609 2.590 FOREWORDOn May 18, 1980, after more than a month of earthquakes and eruptions, Mount St. Helens, in southwestern Washington, exploded in a volcanic eruption more violent than any in the conterminous United States during the 20th century. A lateral blast ofhot gas and rock particles devastated an area of about 150 square miles on the northern side of the mountain knocking down trees to a distance of 15 miles. Several minutes later, a giant ash cloud rose to about 60,000 feet. Winds then carried the ash cloud across the United States, with heavy fallout and deposition in eastern Washington and parts of Idaho and Montana. Earlier, smaller eruptions deposited ash in western Washington and parts of Oregon and Canada.The hydrologic effects of the May 18 eruption have been both widespread and intense. During the eruption, a massive debris avalanche moved down the north flank of the volcano depositing about 3 billion cubic yards of rock, ice, and other materials in the upper 17 miles of the North Fork Toutle River valley. The debris deposits are about 600 feet thick in the upper reaches of the valley. Following the avalanche, runoff from the melted glaciers and snow, and possible outflow from Spirit Lake, caused an extraordinary mudflow in the North Fork Toutle River. The mudflow shattered and uprooted thousands of trees, destroyed most of the local bridges, and deposited an estimated 25,000 acre-feet of sediment in the Cowlitz River channel. A considerable amount of additional sediment was conveyed through the lower Cowlitz into the Columbia River where it was deposited and formed a shoal that blocked the shipping channel. Mudflows also occurred in the South Fork Toutle River and in tributaries on the east flank of Mount St. Helens which enter Swift Reservoir.As part of a concerted Geological Survey effort to study the volcanic event and to identify potential hazards, Survey hydrologists have mounted an intensive program to document the hydrologic effects of the eruptions. The major initial hydrologic findings are reported in this circular series. Quick, useful assessment was made possible only because the Survey has long conducted extensive waterresources investigations in the affected areas of Washington, Oregon, and Idaho. Hence, there was a well-defined basis for identification and documentation of the types and magnitudes of hydrologic changes.The Geological Survey Circular 850, "Hydrologic Effects of the Eruptions of Mount St. Helens, Washington, 1980," consists of individually published short chapters that emphasize data collection activities, field observations, and initial comparisons of pre-and post-eruption conditions. The series will cover hydrologic events occurring on May 18 in the Toutle and Cowlitz River; physical alteration of the Toutle River system; the chemical and physical quality of precipitation, streams, and lakes affected by volcanic ash fall; ...

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