Loads and yields of nutrients and suspended sediment in the Susquehanna River basin, 1985-89

The Susquehanna River, which drains an area of 27,510 square miles in New York, Pennsylvania, and Maryland, is the largest tributary to Chesapeake Bay. Nutrients and sediments from the river are suspected of contributing to a marked decrease in the productivity of the bay. A disproportionately large percentage of these nutrients and sediments originate in the lower part of the Susquehanna River Basin. The lower Susquehanna River Basin is a 5,439-square-mile area between the confluence of the West Branch Susquehanna River at Northumberland, Pa., and the mouth of the river below Conowingo, Md., excluding the Juniata River Basin. Three large hydroelectric dams span the river, at Safe Harbor (Lake Clarke) and Holtwood (Lake Aldred) in southern Pennsylvania, and at Conowingo (Conowingo Reservoir) in northern Maryland. The drainage area of the river at Conowingo Dam is 27,100 square miles. The annual loads (1985-89) of total nitrogen, total phosphorus, and suspended sediment transported from the upper Susquehanna (the area above Northumberland including the West Branch Susquehanna River Basin) and the Juniata River Basins combined averaged 86.6 million, 5.11 million, and 3,170 million pounds. Total nitrogen, total phosphorus, and suspended-sediment loads entering the reservoir system averaged 135 million, 7.71 million, and 5,040 million pounds per year. Most of the nitrogen (131 million pounds), about 60 percent of the phosphorus (4.40 million pounds), and about 25 percent of the sediment (1,400 million pounds) that entered the reservoirs were discharged to Chesapeake Bay. Average yields (1985-89) for total nitrogen, total phosphorus, and suspended sediments in the upper Susquehanna and Juniata River Basins combined averaged 6.1 Ib/acre, 0.36 Ib/acre, and 224 Ib/acre (pounds per acre) per year and yields in the lower basin averaged 15.1 Ib/acre, 0.81 Ib/acre, and 582 Ib/acre per year. Multiple-regression equations developed from relations between land use and measured loads predict annual yields of total nitrogen for a 100-percent urban basin to be 51.6 Ib/acre, for a 100-percent agricultural basin to be 37.1 Ib/acre, and for a 100-percent forested basin to be 1.78 Ib/acre. The annual sediment yield predicted for a 100-percent urban area is 1,960 Ib/acre; for a 100-percent agricultural area, 1,100 Ib/acre; and for a 100-percent forested area, 91 Ib/acre. Yields of total nitrogen, total phosphorus, and suspended sediment for the Susquehanna River Basin upstream from the reservoir system are probably about 3 to 4.5 times the yields prior to the clearing of forests and development in the basin during the 1800's. In addition, the total nitrogen, total phosphorus, and suspended-sediment yields for the upper basin are probably about two to four times the yields when the basin was forested, and yields for the lower basin are probably about seven to eight times the yields prior to early development. INTRODUCTION The District of Columbia and the States of Pennsylvania, Maryland, and Virginia have agreed to a 40-percent reduction in controllable nutrient loads to Chesapeake Bay by the year 2000. Nutrients and sediments that enter Chesapeake Bay by way of the Susquehanna River are suspected of contributing to a marked decrease in the productivity of the Bay. A disproportionately large percentage of these nutrients and sediments are thought to originate in the lower part of the Susquehanna River Basin (Clark and others 1973,1974; Lang, 1982). As part of the Chesapeake Bay Program (U.S. Environmental Protection Agency, 1994), the Bureau of Soil and Water Conservation of the Pennsylvania Department of Environmental Protection (PaDEP), the Susquehanna River Basin Commission (SRBC), and the U.S. Geological Survey (USGS) cooperated in a study to quantify nutrient and suspended-sediment transport and yields in the Susquehanna River Basin. Purpose and Scope This report describes the loads and yields of nutrients and suspended sediment originating in the Susquehanna River Basin. In addition, relations between nutrient and suspended-sediment yields and land use are examined. These relations are used to estimate loads transported to Chesapeake Bay prior to clearing of forest and development in the basin during the 1800's. The data for this report were collected during 1985-89. Loads and yields are given for 16 sites on the Susquehanna River and tributaries that drain the lower Susquehanna River Basin. This report also documents loads of nutrients and suspended sediment transported to the reservoirs on the Susquehanna River and loads transported to Chesapeake Bay. In this report, the average load or yield for the period of data collection is not necessarily the same as the long-term average annual loads and yields reported by Ott and others (1991). Description of the Susquehanna River Basin The Susquehanna River (fig. 1) drains an area of 27,510 mi2 in central New York, central Pennsylvania, and a small part of Maryland. It is the largest tributary to Chesapeake Bay. The long-term mean streamflow of the river where it enters the bay is nearly 40,000 ft3 /s. The upper Susquehanna River Basin is a 22,071-mi2 area that includes the area above the confluence of the West Branch Susquehanna River and the main stem at Northumberland, Pa., as well as the Juniata River Basin. The lower Susquehanna River Basin is a 5,439-mi2 area between the confluence at Northumberland, Pa., and the mouth of the river below Conowingo, Md., excluding the Juniata River Basin. Three large hydroelectric dams span the river, at Safe Harbor (Lake Clarke) and Holtwood (Lake Aldred) in southern Pennsylvania and at Conowingo (Conowingo Reservoir) in northern Maryland (fig. 1). The drainage area at the Conowingo Dam is 27,100 mi2 . The reservoirs behind the dams have trapped large quantities of sediments, nitrogen, and phosphorus. Lake Aldred and Lake Clarke reached equilibrium with incoming river sediments by 1950, and they are no longer storing sediment. Conowingo Reservoir is currently storing sediment; it will probably reach an equilibrium and cease to store sediments within the next 20-30 years (Hainly and others, 1995, p. 37). Woodland accounts for 63.1 percent of the area in the Susquehanna River Basin; tilled cropland, 19.4 percent; pastureland, 6.7 percent; urban land, 9.3 percent; and water, 1.47 percent (Hannawald, 1989). Average annual manure production in the basin is 0.91 ton/acre for the total basin area. Woodlands are concentrated in the northern and western parts of the basin, and agriculture (including manure production) and urban land are concentrated in the lower basin.