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low number of impact craters attests to the
geologic youth of the surface. The western
hemisphere is dominated by a dense concen-
tration of pits crisscrossed by ridges, dubbed
the cantaloupe terrain. The eastern hemi-
sphere consists of a series of much smoother
units, including calderalike structures. These
structures appear to be frozen lakes and are
surrounded by successive terraces indicative
of multiple episodes of flooding and collapse.
The numerous, dark northeast-trending
streaks seen on Triton's south polar cap are
similar to wind streaks on Mars. However,
some Voyager scientists doubt that Triton's
tenuous atmosphere, exerting only 1/100,000
the atmospheric pressure at sea level as that
on Earth, is sufficiently dense to entrain parti-
cles from the surface. These scientists propose
that the streaks are the result of geyserlike
venting of gas particles (fig. 2). Triton's nitro-
gen frost migrates from pole-to-pole every 80
years as the subsolar latitude varies +50°:
therefore, the dark streaks are probably less
than 80 years old. The presence of more than

Figure 2. Profile views of Triton's active
geyserlike plumes. These views are of
regions in the southern portion of Triton
(south is up, west is to the right). The
plumes are about 5 miles high, and the
east-west dimension of each view is about

93 miles.

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100 such streaks suggests ongoing venting activity. Other conclusive evidence exists for active venting. By reprojecting and coregistering images acquired at different viewing angles, a match was found for all of the features except two sets of long westwardtrending dark streaks. The offsets between the paired images of the streaks indicate that these materials are located about 5 miles above the surface. Closer inspection of the images reveals vertical eruption columns extending from the surface to the eastern ends of the streaks. Apparently, these are active geyserlike eruptions in which plume material rises vertically for about 5 miles before being carried downwind above the transition zone between the troposphere and the stratosphere. Images of Triton's complex surface became the highlight of the Voyager and Neptune flyby (figs. 3 and 4). About 50 highresolution images of Triton were acquired during a complex sequence that commenced 8 hours prior to the close flyby of the moon on August 25, 1989. These images were taken through various color filters, at different resolutions, and from rapidly changing spacecraft positions. The computerized processing of such a data set is complex, and, in the past, months or years have been required to produce highquality digital cartographic mosaics. For the Neptune flyby, however, USGS personnel were able to assemble a suite of highresolution, multispectral, geometrically controlled digital mosaics in less than 3 days, which gave the public their first glimpse at this new world. In addition, the USGS, working in collaboration with the JPL Digital Animation Lab, generated a three-dimensional time-lapse simulation of Triton's bizarre surface as it would be viewed by a spacecraft descending over the surface. Final maps of Triton are being prepared, and the USGS is working with the International Astronomical Union to assign names to the many new features. The USGS also plans to publish a geologic map of Triton. Planetary geologic maps are used to gain an understanding of the processes now active on other planets that may have been active on Earth during its formation. Geologic maps also are essential for future planetary exploration that includes manned or unmanned spacecraft landings.

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Water Resources Investigations

Flooding in the Arkansas,
Red, and Trinity Rivers
By Kenneth L. Wahl

produced record or near-record flooding

during April and May in northeastern Texas, southeastern Oklahoma, western Arkansas, and along the Red River in Louisiana. The flooding was the culmination of an extremely wet winter and early spring. In Oklahoma, the statewide average precipitation for the first 4 months of 1990 was the largest January to April total reported since record keeping began in 1892; the 4-month total exceeded the previous high for the period by about 15 percent. The Dallas-Fort Worth Airport reported total precipitation for January to March of 22.05 inches, 129 percent above normal.

These extremely wet conditions were conducive to extensive flooding: by mid-April, soils were saturated, flows in the principal river systems were already near flood stage, and reservoirs and lakes were at or near capacity. Because of these conditions, two major storm sequences in late April and early

I n spring 1990, unusual amounts of rain

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May produced widespread flooding and caused new record high levels in most major lakes and reservoirs in the area. From April 16–26, a series of slowmoving storms developed along a storm front that was centered over southeastern Oklahoma and extended into northern Texas and western Arkansas. These storms produced more than 8 inches of rain over the area, and more than 15 inches were reported at several locations southwest of Dallas, Tex. The late-April rains, which fell on already saturated soils, produced widespread flooding; many rivers and streams crested on April 25 or 26. On the morning of April 30, as the floods were beginning to recede, an abnormally strong, cold air mass moved across the region. The leading edge of this cold air mass stalled on a line from northern Texas to southwestern Arkansas, and the system remained stationary until May 3. During these 4 days, the warm moist air being lifted over the cool stationary front produced torrential rains that caused record flooding in northern Texas, southeastern Oklahoma, and western Arkansas. Rain continued sporadically through mid-May. Total precipitation from April 15 to May 19 averaged more than 16 inches over the affected

:

Mission

he U.S. Geological Survey

has the principal responsibility within the Federal Government to provide the hydrologic information and understanding needed by others to achieve the best use and management of the Nation's water resources. To accomplish this mission, the Water Resources Division, in cooperation with State, local, and other Federal agencies, Systematically collects and analyzes data to evaluate the quantity, quality, and use of the Nation's water resources and provides results of these investigations to the public. Conducts water-resources appraisals describing the occurremce, availability, and physical, chemical, and biological characteristics of surface and ground water. Conducts basic and problemoriented hydrologic and related research that aids in alleviating water-resources problems and provides an understanding of hydrologic systems sufficient to predict their response to natural or human-caused stress. Coordinates the activities of Federal agencies in the acquisition of water-resources data for streams, lakes, reservoirs, estuaries, and ground water. Provides scientific and technical assistance in hydrologic fields to other Federal, State, and local agencies, to licensees of the Federal Energy Regulatory Commission, and to international agencies on behalf of the Department of State. Administers the State Water Resources Research Institutes Program and the National Water Resources Research Grants Program.

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area and more than 24 inches on the Arkansas-Oklahoma State line. Rainfall recorded at numerous individual precipitation stations, however, greatly exceeded the averaged amounts. New record-high flows occurred at many streamflow gaging stations because of the early May storm. Flooding equal to or greater than the 50-year recurrence interval occurred in streams originating in the areas of greatest precipitation; floods of the magnitude of 100year or greater recurrence-interval floods occurred on many streams. A recurrence interval is the probability of occurrence and the average number of years between occurrences; for example, a 100-year flood, on average, is the flood magnitude that occurs in a 100-year period. Regularity of occurrence, however, is not implied: a 100-year flood might be exceeded in consecutive years or might not be exceeded in a 50-year period. Most major rivers in this area are controlled by dams, many of which have been in place for more than 25 years. The levels of most of the reservoirs upstream from these dams reached new record highs during this period. Because these reservoirs were full before the storm abated, the runoff from the floods could not be contained. As the runoff from the tributary streams reached the major rivers, the flooding that occurred exceeded all previous floods since the major storage reservoirs were completed. The Arkansas, Red, and Trinity Rivers are the principal river systems that had the greatest flooding. Because of the size of the

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area affected by the storms and the saturated condition of the soil, the flooding extended far downstream from the source areas. In addition, the relatively flat slopes of the rivers and the need to reduce the storage levels in the reservoirs increased the duration of downstream flooding. Downstream from Livingston Reservoir on the Trinity River, for example, flood stage usually corresponds to a discharge of about 52 billion gallons per day; the discharge from that reservoir exceeded that amount for 10 consecutive days (May 17–26).

Because the main-stream impoundments, such as Eufaula Lake on the Canadian River (tributary to the Arkansas River), Lake Texoma on the Red River, and Livingston Reservoir on the Trinity River, have large storage capacities, major reductions in flooding would be expected to occur as the flood peaks passed though the storage systems. All three impoundments were full, however, when the April 30 to May 3 storm occurred. In addition, the extremely large inflows to the reservoirs would have taxed the systems even if the pool levels had been normal for early May.

The total maximum daily inflow to Eufaula Reservoir is estimated to have exceeded 259 billion gallons per day. On 1 day alone, about 800,000 acre-feet (an acre-foot is the volume of water that covers an acre of land to a depth of 1 foot) of water entered the reservoir; this is about 20 percent of the total capacity of the reservoir. The inflow to Livingston Reservoir was greater than 200,000 acre-feet (about 10 percent of capacity) per day for 7 consecutive days (May 10–16). Maximum daily inflow to Lake Texoma was also about 10 percent of total reservoir capacity.

A separate and somewhat isolated thunderstorm during May 19–20 produced 13

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inches of rain in about 8 hours and caused

Overflow of Lake Texoma on May 7, 1990. Spillway overflow channel cuts north from the lake (top of

photograph).

severe flooding in Hot Springs, Ark. The resulting flood peak on the Ouachita River (tributary to the Red River) about 20 miles downstream from Hot Springs was about 103 billion gallons per day. This peak was 14 percent greater than the previous record peak that occurred in 1923. Seventeen deaths and millions of dollars in damage to public and private property are attributed directly to the storms and related flooding in the four-State area. Agricultural losses were extensive. In Arkansas and Oklahoma alone, the homes of more than 2,000 families were either damaged or destroyed. Public facilities in the entire area that were damaged or destroyed include roads, bridges, and water and sewage treatment facilities. Before the flooding subsided, 104 counties in the area had been declared eligible for Federal disaster assistance.

Toxic Substances in
Water Resources
By Gail E. Mallard and Paul D. Capel

he USGS examines the fate and trans| port of contaminants in the Nation's waters through the Toxic Substances Hydrology program. Research concentrates on determining the mechanisms and the chemical and biological processes that control the environmental fate of pollutants at selected contaminated field sites. Physical, chemical, and biological reactions change the chemical structure of contaminants. Newly produced (daughter) chemicals can have either greater or lesser toxic effects and mobility in the environment. Discovering how these transformation reactions

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