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The relation between the river system and Their interest in preserving the natural prothe bay contributes directly to the economic ductivity and hydrologic cycles of the welfare of the people surrounding the bay. Apalachicola River and bordering flood plain

This Landsat image, which is a false-color composite obtained on February 11, 1977, shows the extent of the flood plain in the Apalachicola River basin, Florida. The dark color of the flood plain is caused by the low reflectance from flood waters. The 656-footwide river is barely visible in the center of the 2- to 5-mi/e-wide flood plain. The Apalachicola River flows from Lake Seminole lat the top), 106 miles south, to Apalachicola Bay (near the bottom of the scene}. The numerous white squares near the top of the scene are agricultural fields in Florida and Alabama. The large red area east of the

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Research vessel Rockfish.

Potomac River Estuary Study

Estuaries are potentially the most productive as well as the most fragile and endangered areas of our Nation's coastal environment. Because they are the meeting place of saltwater and freshwater, estuaries are complex hydrodynamic, chemical, and biological environments. The fact that they are sinks for sediments further complicates the picture because nutrients, metals, and organic pollutants are often associated with sediments. These substances may become permanently or temporarily stored in the bottom sediments promoting eutrophication (enrichment of the food supply) and, in the case of metals and organics, sometimes concentrating in the life forms. Partly because of their complexity and partly because it is difficult, dangerous, and expensive to study them, estuarine environments are poorly understood. In light of increasing awareness of their importance and fragility, it becomes more and more imperative to make the extra effort to collect and interpret the data necessary to understand estuarine resources. It was in this vein that, in October 1977, the U.S. Geological Survey began its 5-year Interdisciplinary Potomac Estuary Study.

Acceleration of the natural filling process by increased sediment loads from upstream farms and the Washington, D.C., urban area is probably the most visible

water-quality problem in the tidal Potomac River and marginal embayments. This process has been so rapid that the Lincoln and Jefferson Memorials now stand on what was described in 1711 as a harbor suitable for great merchant vessels.

Another persistent water-quality problem in the tidal river is the depletion of dissolved-oxygen associated with the disposal of municipal sewage and storm runoff from the Washington, D.C., metropolitan area. Following the installation of a water supply system for the District of Columbia in 1859, the problem of effluents and runoff was first apparent when large quantities of domestic sewage began entering the river by way of the storm sewers. Soon the loading was beyond the capacity of the river in the waterfront area of the city, and, by 1899, the river had become so obnoxious that President Benjamin Harrison appointed a board of sanitary engineers to find an outfall location where dilution was adequate and tidal action did not return the sewage to the edge of the city. The development of waterquality problems near the District of Columbia coincided with the growth of population. Improvements in water quality paralleled installation of and improvements in waterpollution-control technology used in the Blue Plains sewage treatment plant, the major treatment facility in the metropolitan

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area. Some of the worst observed dissolvedoxygen conditions occurred in the mid- to late 1950's when carbonaceous biochemical oxygen demand loadings were at their peak. Today, the major problem has shifted from that of dissolved-oxygen depletion due to carbonaceous biochemical oxygen demand to one of eutrophication caused, at least in part, by the heavy nutrient loadings from the Washington, D.C., area municipal sewage treatment plants.

Because 72 percent of the 3.9 million people in the Potomac River basin are concentrated in a metropolitan area and because the primary business is government, the Potomac River, unlike many east coast estuaries and tidal rivers, is relatively free from the pollution problems associated with manufacturing and chemical industries. Therefore, the effects of sedimentation and eutrophication can be studied in the tidal Potomac River and Estuary independent of complications from those of other types of pollutants.

The Potomac Estuary Study is one of seven pilot River-Quality Assessments and the only one to concentrate on estuarine problems. Others have concentrated on problems in upland streams such as dissolved oxygen and excessive nutrient enrichment in the Willamette River-Quality Assessment or those associated with energy development and irrigation as in the Yampa Study.

The general objectives of the Potomac Study are (1) to conduct research into the physical, chemical, and biological mechanisms governing life cycles of phytoplankton, submerged vegetation, and benthic fauna in tidal rivers and estuaries, (2) to develop mathematical models necessary to support ecological models suitable for predicting the influence of phytoplankton on dissolved oxygen and nutrient levels (these models are

designed to expedite water-quality management decision making for the tidal Potomac River and Estuary), and (3) to develop, refine, and standardize efficient techniques for studying water quality of the Potomac and other tidal rivers and estuaries.

Examples of some of the many findings of the Potomac Estuary Study are summarized in the paragraphs below.

Low-Flow Dissolved-Oxygen Relationships

Nutrients associated with sediment discharged by the Potomac River during periods of high flow settle to the bottom. They become a major source of dissolved and particulate nutrients in the water column of the freshwater tidal river and estuarine transition zone (see fig. 4 for definition of these zones) during critical periods when river flow is low. During such periods, nutrients released through the 390-million-galIon-per-day discharge from the regional secondary sewage treatment plant cause a dissolved-oxygen sag for approximately 8 nautical miles in the upstream part of the tidal river. In the remainder of the tidal river and in the transition zone, the oxygen regime is dominated by biologic reactions fed by nutrients exchanged with the bottom. During extreme flood events, some of this material is moved to the transition zone. Therefore, the magnitude, frequency, and duration of flood events which transport most of the annual upland nutrient loads may be a major determinant of lowflow water quality in the downstream reaches of the tidal river and transition zone.

Tributary Loads

A sampling program was begun in 1979 to acquire data on monthly and major-storm

Percentage of material supplied during 1980 and 1981 water years and trapped in the various zones of

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Percentage trapped

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the tidal Potomac River and Estuary.

[Negative numbers in parentheses indicate a net outflow of dissolved silica from the transition zone. This is included in the material trapped in the estuary and is, therefore, not subtracted from the total percentage. The percentage of material supplied to the Chesapeake Bay is 100 minus the number shown on the line labeled "Total percentage" ]

w Sediments Dissolved silica Total nitrogen Total phosphorus
ater year
1980 1981 1980 1981 1980 1981 1980 1981

Segment:

Tidal river __________________ __ 38 18 16 35 7 2 5 27

Transition zone _____________ __ 16 30 (—7) (—9) 3 41 16 13

Estuary ____________________ __ 46 43 19 42 51 4 40 40 Total percentage ______________ __ 100 91 35 77 61 47 61 80 Total supply in tons ____________ __2.38><10° 783,000 127,000 43,000 41,700 19,400 2,710 1,490

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loadings of phosphorus, nitrogen, biochemical oxygen demand, dissolved silica, and sediment from nonpoint sources. Samples of the streamflow leaving four watersheds and from the Occoquan Reservoir in Virginia were collected. These data were correlated with discharge records and with reservoir characteristics so that pollutant loads could be estimated. Results from the three urban watersheds sampled indicate that suspended sediment is less than that measured 20 years ago. Data collected from the discharge of the reservoir were compared with available information on the amount of pollutants entering the reservoir. The results indicate that, during 1979 and 1980, most of the sediment and phosphorus was trapped, but most of the nitrogen passed through to the tidal Potomac River.

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freshwater, and environmentally impacted. Analysis indicated that salinity is the predominant factor influencing infaunal distributions throughout most of the tidal river. Biological communities that characterize the four zones are primarily depositfeeding resilient opportunists. Within the estuarine zone reside marine organisms that tolerate a wide range of salinities and that recolonize this area which is seasonally influenced by summer anoxic conditions. Estuarine endemics dominate the transitional zone where seasonal salinity fluctuations decrease diversity while supporting increased abundance. Freshwater species dominate the remaining reach of the river.

Nutrient and Sediment Budgets

During the 1980 and 1981 water years, twice-weekly samples of salt, sediment, and a variety of nutrients were collected at five stations along the 116-mile tidal reach between Washington, D.C., and the Chesapeake Bay. Computed nutrient and sediment loads passing each of the stations were used in combination with sewage treatment plant and local nonpoint source contributions to determine materials budgets for the major segments of the tidal river and estuary for

ln-situ incubation of bottles for primary productivity measurements in the tidal Potomac River.

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