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

Abstract

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.