interesting aspects of the organic pollutants. Some pollutants are found only in certain rivers. For example, fire retardants found in the Illinois River and a triazine-trione compound (an industrial chemical) found in the Ohio River are absent from other rivers and can be used as chemical fingerprints to trace the waters of the Illinois and Ohio through the Mississippi system. Atrazine and other herbicides are ubiquitous throughout the entire Mississippi system. Currently, 100 tons of atrazine per year, or 0.2 percent of the entire U.S. production, is estimated to be carried by the Mississippi River to the Gulf of Mexico. Catfish in the river contain significant levels of DDT and its metabolites in their fatty tissue. Even though the use of this insecticide has been banned in the United States since 1972, enough DDT and metabolites remain stored in river sediments to be ingested by the bottomfeeding catfish.

The Mississippi now carries to the Gulf of Mexico only about half the

sediment it carried
before 1950.

In order to gain experience and refine sampling techniques with minimal hazard to personnel and equipment, the first three sampling trips were made during periods of low river flow. Now that procedures are fairly well set, the next few sampling trips will be scheduled for periods when the river is expected to be deep and swift and when large quantities of sediment will be in transport. Over the next several years, other phases of river activity, such as rising stage and falling stage, will be sampled to achieve an understanding of how pollutants are transported and stored, and how these processes vary from one season to the next during an average river year. With this knowledge, we will be better equipped to assess the condition of the Mississippi River.

Drought in the United States, 1987-88

By Thomas G. Ross

Drought occurred in the Southeast in 1986 and in the Pacific Northwest and much of California in 1987. Drought is again occurring in those areas and in other areas of the United States. Data collected by many agencies indicate below-normal precipitation, streamflow in the below-normal range, and low reservoir contents in the affected areas. Municipal water supply, stream water quality, agriculture, recreation, fisheries, navigation, and power generation all suffered adversely from the effects of the latest drought.

Just how severe is the drought of 1987-88? If measured in terms of streamflow, the severity of the drought can be assessed in terms of how low streamflow becomes (intensity), how widespread is the low streamflow (areal extent), and how long the low streamflow persists (duration). The amount of water stored in reservoirs is also an indicator of drought severity.

Drought Intensity

As a general measure of streamflow conditions, the combined flow of the three largest rivers in the lower 48 States-Mississippi, St. Lawrence, and Columbia-set a record low average June flow of 481 billion gallons per day (bgd), which is 45 percent below median. (Median streamflow is the middle value of monthly flows that occurred in a given month during the 1951-80 reference period. That is, streamflow was below this median value in 15 of the 30 years and above this value in the other 15 years.) The combined average flow of the rivers dropped 22 percent from May to June. Combined flow for June 1988, the lowest June flow of record, was less than 1 percent lower than the June 1934 flow, which was the previous June low for the period of combined record going back 59 years; the widespread extent of the current drought is shown by its effect on the three major U.S. river basins.

The intensity of the drought also is indicated by record low flows during June 1988 at 33 streamflow index stations in southern Canada and the conterminous United States. Thirty-one of these sites are on or east of the Mississippi River, including the Mississippi River at Vicksburg, Miss. On that river, the largest in the United States, the monthly average discharge of 139 bgd was 62 percent below median and 2 percent below the previous low for June, which occurred in 1934.

At the station with the longest period of record (99 years) of the 33 stations, Chippewa River at Chippewa Falls, Wis., June streamflow averaged 748 mgd, 79 percent below median and 23 percent below the previous low flow for June, which also occurred in 1934. Of the record low flows for June, 14 broke records set in the "Dust Bowl" years (1930-36).

Areal Extent and Duration

Areal extent and duration are two other measures of drought that must be taken into account. Streamflow maps for southern Canada and the conterminous United States for water years 1986, 1987, and 1988 (a water year covers the period October 1 to September 30) show both the areal extent and the duration of below-normal-range streamflow (fig. 3A-C). Streamflow conditions for May through September 1988 (fig. 3D-H) emphasize change during those months. Streamflow was in the below-normal range over about 38 percent of the area during May 1988, over about 60 percent of the area during June 1988, over about 50 percent of the area during July 1988, over about 39 percent of the area during August 1988, and over about 27 percent of the area during September 1988.

An overall measure of the duration of dry conditions is the departure of actual streamflow from the 1951-80 reference period median flow. The percent departure of actual flow from median since October 1983 for the 181 index streamflow stations in southern Canada and the United States (fig. 4A) shows that streamflow was above median for every month through April 1985, generally above median from May 1985

Figure 4. Percent departure of actual monthly average streamflow from the 1951-80 reference period median flow for the period October 1, 1983-September 30, 1988. (Zero represents median flow for the reference period.)

through December 1986, and generally below median since that time. Regional graphs (fig. 4B-F) show streamflow variability in five areas severely affected by the 1988 drought: California, the Pacific Northwest, the Northern Great Plains, the Western Great Lakes, and the Southeast.

California-Streamflow at the six index stations has been below median in 31 of 43 months since January 1985, and continuously from August 1987 through June 1988 (fig. 4B). Streamflow at the index stations in 1988 has ranged from 38 percent above median (July) to 65 percent below median (March). No new record low flows have occurred since March. About 54 percent of the total annual runoff in California usually occurs from December through March, the highest in February. February streamflow was 71 percent below normal. • Pacific Northwest (Montana, Idaho, Washington, and Oregon)-Drought had persisted for 18 consecutive months as of the end of September 1988 (fig. 4C). Above-average precipitation during April increased streamflow at the 17 index stations to 6 percent below median, but June streamflow decreased to 42 percent below median. Late spring and summer streamflow in the Pacific Northwest is highly dependent on precipitation during the preceding December to February period. Departure from normal of precipitation for that period in 1987-88 was highly variable in the four States and, based on a few National Weather Service index sites, was as follows: Montana, 19 to 73 percent below normal; Idaho, 15 to 46 percent below normal; Washington, 37 percent above normal to 34 percent below normal; and Oregon, 1 to 37 percent below normal. Actual streamflow for the last 12 months was much less than median until April 1988. Median stream

flow in the Pacific Northwest usually increases from March through June due to snowmelt. Even if the snowpack is below normal, increases in streamflow can be caused by above-average precipitation in the form of rain, as was the case during April. Since the snowpack was generally below normal in the four-State area during the 1987-88 winter, any significant sustained increase in streamflow had to come from rainfall in excess of that required to satisfy soil-moisture deficits. • Northern Great Plains (the Dakotas, Nebraska, Minnesota, and Iowa)-Total streamflow at the 16 index stations generally was well above median through March 1987 (fig. 4D). Conditions at the index station Mississippi River at St. Paul, Minn., are an example of what has occurred in the five-State area since March 1987: In April 1987, flow that had been in the above-normal range from May 1985 through March 1987 decreased into the below-normal range, and it has been only as high as the normal range for 7 of the last 18 months (through September 1988). During water year 1988, streamflow was below median for 9 months and consecutively from March through September.

Western Great Lakes (Wisconsin, Illinois, Indiana, Michigan, and Ohio)Total streamflow at the 18 index stations generally was above median through December 1986 (fig. 4E). Since then, flows have been below median during 15 of 21 months, and consecutively from March through September 1988. The June 1988 flow of the Chippewa River at Chippewa Falls, Wis., was cited earlier as an example of record low flows for June, but record low flows for June also occurred at 10 other index stations in the region.

• Southeast-Actual streamflow has been below median for 14 of the last 17 months, 11 of them consecutive through August 1988 (fig. 4F). The Southeast includes those States in the area from the Mississippi River to the Atlantic Ocean and south of the Ohio River and the Pennsylvania State Line. Total streamflow at the 39 index stations in the Southeast has been at least 24 percent below median since October 1987 and was 55 percent below median for June 1988. In 8 of 12 months of the 1988 water year, streamflow has been lower than the streamflow in the comparable months of water year 1986. Figure 4F clearly shows the 1986 drought as 10 consecutive months of below-median streamflow and the 1988 drought as 11 consecutive months of below-median streamflow. During the current dry period, 17 new monthly low flows occurred at index stations from June 1987 through April 1988, 9 of them during March 1988, at sites from West Virginia to Alabama. There were no new low flows during May, but 16 new low flows occurred in the area during June. For example, streamflow of the French Broad River near Asheville, N.C. (92 years of record), was 23 percent below the previous June low, set in 1925, and 69 percent below median for June.

Contents of Reservoirs

Many areas of southern Canada and the conterminous United States are dependent on water stored in reservoirs for agricultural, municipal, and industrial water supply; for power generation; for recreational use, from water sports to fishing; and for maintenance of streamflow for fish migration. The cumulative

effects of many months of below-normalrange streamflow are reflected in the amount of water stored in many reservoirs nationwide. Monthend index reservoir contents for September 1988 were in the below-average range at 34 of 100 reporting sites, including most reservoirs in New Jersey, North Dakota, Montana, Wyoming, Idaho, California, and Nevada. September 1988 contents were lower than those of September 1987 at 49 of the 100 sites, including most reservoirs in New Jersey, the Tennessee Valley, Nebraska, the Dakotas, Montana, Wyoming, California, Nevada, Utah, Colorado, and New Mexico. In the Southeast, 3 of the 10 index reservoirs with capacities greater than 1,000,000 acrefeet (an acre-foot is the amount of water necessary to cover 1 acre of land with water 1 foot in depth or about 326,000 gallons) had contents that were below those of September 1986.

Summary

Intensity, areal extent, and duration— record low flows at 33 index stations during June, 60 percent of the area of southern Canada and the conterminous United States with streamflow in the below-normal range, and many months of below-median flow-these three criteria define the current drought as severe in some areas through September 1988. Are we in for a repeat of the “Dust Bowl" years? The current drought has not had the duration of the drought of the 1930's, but many record low flows that occurred during the 1930's (and later and shorter drought episodes of the mid-1960's, 1977, and 1981) have been broken by new low flows set since 1985. Only weather and time will tell.