Sediment: 
For well over 200 years, excessive erosion and subsequent sediment deposition have been a major Anacostia River problem. High sedimentation rates, associated with early tobacco growing in the 17th and 18th centuries, necessitated the first dredging of the Anacostia River in 1804. Continued high sediment deposition in the tidal river ultimately led, by 1830, to the demise and abandonment of Bladensburg as a major Atlantic seaport (Wright, 1977). Because the Anacostia River functions in many ways like a tidal lake, it is a very efficient sediment trap. It has been estimated that approximately 85 percent of the incoming sediment load remains trapped within the river (Scatena, 1986). This has necessitated frequent and costly sediment removal to maintain marina areas and navigation channels (Figure 6). In addition to adversely impacting navigation, reducing water clarity, degrading aquatic habitat and associated biota, sediment serves as a binding site for a broad range of urban pollutants and toxicants. These include: petroleum hydrocarbons, trace metals such as lead, mercury, cadmium, copper and zinc, PCBs, pesticides, herbicides, nutrients and bacteria.

Sediment-related stream quality degradation in the non-tidal portion of the Anacostia has been equally profound. Related impacts include: impairment of riffle and pool habitat through deposition of fine sediments such as sand, silt and clay; accelerated streambank and streambed erosion during stormflows; and high suspended solids loads which impair the biological community by obscuring the water for sight feeders and clogging or irritating exposed gills.

Using general suspended sediment-watershed area curves (Schueler, 1987), COG staff estimated annual total suspended solids loads (TSS) generated in the Anacostia watershed. As seen in Figure 7, the two largest subwatersheds (Northwest and Northeast Branch) each contribute the largest total TSS loads. Not surprisingly, TSS loads are generally a function of drainage area and land use, with the largest subwatershed and/or most highly developed one contributing the largest load. TSS loads for the intensively developed Lower Beaverdam Creek subwatershed are the highest per unit area in the watershed. Annual TSS loadings in the Anacostia watershed are estimated to be 48,200 tons, for an average of 0.43 tons/acre/year (Warner et al., 1997).

Nutrients
In freshwater ecosystems, two nutrients, phosphorus and nitrogen, can significantly impact receiving waters. When present in sufficient concentrations they often trigger algal blooms, which eventually reduce the dissolved oxygen (DO) level of the water as decaying algal and other organic matter is broken down by microorganisms. Typical sources of phosphorus and nitrogen include fertilizers, animal wastes, automotive exhaust, organic material, soil, etc.  

Using the Simple Method (Schueler, 1987), COG staff estimated total phosphorus (TP) and total nitrogen (TN) loads for the entire Anacostia watershed. As expected, the larger Northeast Branch portion of the watershed generates the largest TP and TN loads (50,000 and 340,000 lbs/year, respectively). However, when viewed on a pollutant load per acre basis, it is evident that the Lower Tributaries and Tidal Anacostia areas contribute disproportionately to the overall problem. Much of this is due to the high amount of impervious surface, low number of storm water management controls, age of sanitary sewer lines, storm drainage and combined sewer systems present.

Combined Sewer Overflows-CSOs
Approximately 60 percent of the Anacostia watershed within the District of Columbia drains directly to the tidal Anacostia River via a combined sanitary and storm sewer system dating back as early as the late 1800s. There are 11 major combined sewer outfalls to the Anacostia River and all discharge in the vicinity of the East Capitol Street and South Capitol Street bridges. A CSO event occurs when rainfall exceeds the capacity of this combined system causing discharges of untreated sanitary waste and storm water directly into the river. On average, overflows occur roughly 40 to 50 times a year, resulting in approximately 1.3 billion gallons of sanitary waste discharged to the tidal river.

CSOs are the primary point source pollutants degrading the Anacostia River's water quality. However, only about six percent of the annual pollutant loads to the Anacostia River are from CSOs compared to about 94 percent from nonpoint sources (Warner et al., 1997). In recognition of the CSO problem, the District of Columbia initiated its CSO Abatement Program in the early 1980s (Nemura and Pontikakis-Coyne, 1991). Retrofitting of existing combined sewer systems, between 1988 to 1990, with inflatable dams and construction of an overflow treatment facility (a.k.a., swirl concentrator) have produced some improvement. However, it is estimated that well over $1 billion dollars may be required to correct the existing CSO problem.

Toxics
Toxics refer to a variety of contaminants including trace metals such as arsenic, mercury, copper, cadmium and lead; and organic compounds such as PAHs (polycyclic aromatic hydrocarbons), PCBs (polychlorinated biphenyls) and pesticides and herbicides (e.g., DDT, Chlordane and atrazine) which reach receiving waters from storm water runoff, atmospheric deposition and industrial and municipal discharges.

These contaminants typically cling to particles suspended in water and settle to the bottom, whereupon, they can become ingested by bottom feeding organisms and potentially find their way up the food chain. As seen in Figure 9, the Anacostia River is one of three areas in the Chesapeake Bay recognized as posing a significant risk to aquatic life due to high levels of sediment contamination. It has been designated by the Chesapeake Bay Program as a "Region of Concern" and the District of Columbia Department of Consumer and Regulatory Affairs has developed an "Action Plan" to address the issue of toxics in the river (CBP, 1995).

Several studies of tidal river sediments have found PCBs, DDT, DDE, Chlordane, trace metals and PAHs at detectable levels at all tidal Anacostia River sampling stations with levels of PCBs and Chlordane exceeding suggested criteria throughout the tidal river.

However, the source(s) of contaminants could not be definitively determined (LTI, 1990). A subsequent study of tidal river sediments conducted in 1991 found concentrations of trace metals, such as cadmium, mercury, lead and zinc in the vicinity of the Washington Navy Yard to be at levels several times those expected to occur naturally. In addition, the study also found sharp increases in two organic sediment contaminants, DDT and PCB, just downstream of the Navy Yard. Potential sources identified include: the Navy Yard (Figure 10), the Bureau of Engraving and Printing, the old Lionel freight yard and the U.S. Botanical Gardens. In contrast, concentrations of Chlordane were highest in and just downstream of Kingman Lake.

Currently, a major initiative, led by the District of Columbia Department of Health, is underway to study both toxic loadings to the tidal river from the Northwest and Northeast Branches, as well as, the control of and management of contaminated river sediments. Because of elevated levels of PCBs and Chlordane found in fish tissue, the District of Columbia, in 1994, issued a fish consumption advisory recommending that no bottom dwelling fish (such as catfish, eel and carp) be consumed and no more than 0.5 pounds of game fish (such as largemouth bass and sunfish) be consumed by an adult per week. This advisory remains in effect.

Organic Loadings
Storm water runoff, combined sewer overflows, leaking sewer lines, as well as natural processes, all contribute significant amounts of organic matter to the Anacostia River. Organic matter, which refers to anything derived from living organisms, must then be broken down or decomposed by microorganisms within the river. Depending on the timing and size of the load, the decomposition of this material can require a substantial amount of oxygen. One measure of the amount of oxygen required to decompose organic matter (principally organic carbon) over a fixed amount of time (typically 5 days) is termed the five-day biochemical oxygen demand (BOD5). While BOD5 does not account for the total oxygen demand to a water body, it does provide a good representation. When characterized as a pollutant load, BOD5 is expressed in terms of the total organic load to a receiving waterbody that is biologically oxidizable.

Again, using the Simple Method (Schueler, 1987), COG staff estimated BOD5 pollutant loads for the entire Anacostia watershed. As seen in Figure 11, the Northwest and Northeast Branches (which together comprise approximately 74 percent of the total Anacostia watershed area) generate roughly 72 percent of the watershed's BOD5 loads. In general, BOD5 pollutant loads per subdrainage area increase with increasing subdrainage area size. The total BOD5 pollutant load for the entire watershed is an estimated 2,915,680 lbs/year. This level is approximately 5 to 6 times higher than under pre-European settlement conditions.

High BOD₅ loads, particularly during the warmer summer months, can reduce tidal river dissolved oxygen (DO) concentrations to levels that are lethal to fish and other aquatic organisms. Other factors that influence DO concentrations include river flow, water temperature, CSO events, algal blooms and sediment oxygen demand (SOD). SOD has been found to have a major negative influence on DO within the tidal Anacostia River, particularly in the vicinity of CSO outfalls (An, 1992). The District of Columbia has established a minimum DO concentration of 5.0 mg/L to support aquatic life. Unfortunately, chronically low DO levels below this threshold have been and continue to be a major problem along the tidal river (Figure 12). For a significant portion of the tidal river, from below Kenilworth Marsh downstream to approximately the South Capitol Street bridge, minimum summer DO levels were at or below 1.0 mg/L for the years 1987 to 1990 (Herson-Jones et al., 1994). While only two continuous DO monitoring stations have remained operational since 1990, data through 1996 show that very low summer DO levels are still a common occurrence. 

Despite low DO levels, the number of fish kills reported in the tidal river over the past 10 years has generally been declining. The District of Columbia's Fisheries Management Branch which investigates reports of fish kills within the Anacostia River roughly defines a fish kill as the death of approximately 50 or more individuals within a spatially confined area (Tilak, 1997). From 1990 to 1996, the Fisheries Management Branch has observed two fish kills, one in June 1991 and the other in June 1992. Extremely low DO levels were believed responsible for the 1991 fish kill in which at least 10 fish species were identified. DO levels associated with the fish kill ranged from 0.4 to 1.8 mg/L (Tilak, 1997).