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Figure 7-3 Long-term trends in mean, tenth, and ninetieth percentile streamflow in summer (July-September) for the Delaware River at Trenton, New Jersey (USGS Gage 01463500). Source: USGS, 1999.

reservoir releases, regulated to maintain a minimum flow of 2,500 to 3,000 cfs at Trenton, can be greater than 60 percent of the inflow to the estuary (Albert, 1997).

The Delaware River-Delaware Bay system is one of the major coastal plain estuaries of the East Coast of the United States. The tidal river and estuary extend a distance of 134 miles from the fall line at Trenton, New Jersey, to the ocean mouth of Delaware Bay along an 11-mile section from Cape May, New Jersey, to Cape Hen-lopen, Delaware (Figure 7-2). Because Philadelphia is a major East Coast port, a navigation channel is maintained to a depth of 12 meters (39 feet) from the entrance to the bay upstream to Philadelphia. From Philadelphia to Trenton, the channel is maintained at a depth of 8 meters (26 feet) (Galperin and Mellor, 1990). The semidiurnal tide has a mean range of 1.5 meters (4.9 feet) at the mouth of the bay and propagates upstream on the incoming tide to Trenton in approximately 7 hours; typical tidal currents are approximately 1.5 meters/second (Galperin and Mellor, 1990). Approximately 25 miles downstream from Philadelphia, tidal currents near the Tacony-Palmyra Bridge (RM 107) are characterized by vigorous vertical mixing and a marked current reversal with currents of approximately 1.0 meter/second (Thomann and Mueller, 1987).

The Delaware estuary can be characterized as three distinct hydrographic regimes based on distributions of salinity, turbidity, and biological productivity: (1) tidal freshwater, (2) transition zone, and (3) Delaware Bay zone. The tidal fresh river extends about 55 miles from the head of tide at Trenton, New Jersey (RM 134) to Marcus Hook, Pennsylvania (RM 79). Under mean freshwater flow conditions, salinity

Figure 7-4 Monthly trends of mean, tenth, and ninetieth percentile streamflow for the Delaware River at Trenton, New Jersey (USGS Gage 01463500), 1951-1980. Source: USGS, 1999.

intrusion in the tidal fresh section of the river generally extends upstream to the reach between the Delaware Memorial Bridge at Wilmington (RM 68.7) and Marcus Hook (RM 79.1). During drought periods (e.g., 1962-1966), salinity intrusion is a concern because industrial water withdrawals are less desirable and recharge areas of the South Jersey aquifers serving the Camden metropolitan area are potentially threatened (DRBC, 1992). During drought conditions, the Delaware River Basin Commission requires releases from the upper basin reservoirs to prevent critical salinity concentrations from intruding farther upstream than Philadelphia at RM 98 (DRBC, 1992).

The transition zone, extending about 26 miles from Marcus Hook (RM 79) to Artificial Island, New Jersey (RM 53), is characterized by low salinity levels, high turbidity, and relatively low biological production (Marino et al., 1991). The estuarine region extends downstream of Artificial Island about 53 miles to the mouth of Lower Delaware Bay; salinity in this region varies from approximately 8 ppt upstream to approximately 28 ppt at the mouth of the bay (Marino et al., 1991).

POPULATION, WATER, AND LAND USE TRENDS

Four densely populated metropolitan areas have developed along a 50-mile industrialized section of the Delaware River from Philadelphia, Pennsylvania, and Trenton and Camden, New Jersey, to Wilmington, Delaware. From 1880 to 1980, urban growth accounted for most of the 236 percent increase in total population of the region. The urban proportion of the population increased from approximately 64 percent at the turn of the twentieth century to approximately 80 percent in 1980 (Marino et al., 1991). During the period after World War II, from 1950 through 1980, development in the region was characterized by urban and suburban sprawl: urban land use area increased from 460 square miles in 1950 to 3,682 square miles by 1980, while population density declined from 8,000 to 3,682 persons per square mile (Marino et al., 1991). Much of this development occurred by converting agricultural lands in close proximity to the major metropolitan areas to suburban land uses.

The Delaware River case study area includes 14 counties identified in 1995 by the Office of Management and Budget (OMB, 1999) as the Philadelphia-Wilmington-Atlantic City, PA-NJ-DE-MD Consolidated Metropolitan Statistical Area (CMSA) (Table 7-1). Long-term population trends from 1940 through 1996 for these counties are presented in Figure 7-5. Population in these counties has increased by 162 percent, from 3.67 million in 1940 to 5.97 million by 1996 (Forstall, 1995; USDOC, 1998).

The city of Philadelphia withdraws water for domestic water supply at the Torres-dale intake upstream of the salt front. The city of Trenton also withdraws water for public water supply from the Delaware River. In addition to these cities, Camden, the Delaware County Sewer Authority, and Wilmington are among more than 80 dischargers of municipal wastewater directly to the estuary or the tidal portions of its tributaries. Historical water use data are not readily available at the county level of aggregation to assess the contribution of the Delaware estuary region to municipal and industrial water withdrawals compiled by the USGS for the entire Middle-Atlantic Basin from 1950 to 1995 (Solley et al., 1998). The natural resources (plankton, fisheries, marshes, and shorebirds), human uses (waste disposal, transportation and dredging, beach development), and management issues of the Delaware estuary are presented in Bryant and Pennock (1988).

TABLE 7-1 Consolidated Metropolitan Statistical Area (CMSA) Counties in the Delaware Estuary Case Study, Philadelphia-Wilmington-Atlantic City, PA-NJ-DE-MD CMSA

New Castle County, DE Cecil County, MD Atlantic County, NJ Burlington County, NJ Camden County, NJ Cape May County, NJ Cumberland County, NJ Gloucester County, NJ Salem County, NJ Bucks County, PA Chester County, PA Delaware County, PA Montgomery County, PA Philadelphia County, PA

Source: OMB, 1999.

Figure 7-5 Long-term trends in population for the Philadelphia-Wilmington-Atlantic City CMSA counties of the Lower Delaware River-Delaware Bay. Sources: Forstall, 1995; USDOC, 1998.
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