Acid rain is a concern. This normally spikes in spring. With the increase in acid, goes the increase in soluability for a lot of things. In the east this sort'a offsets the decrease in soluability due to colder temps.
Most of the pollution in the river is due to excess animal manure from farming, agricultural runoff, urban and suburban stormwater runoff, and raw or inadequately treated sewage. In 2003 the river alone contributed 44% of the nitrogen, 21% of the phosphorus, and 21% of the sediment flowing into the Chesapeake Bay. It was designated as one of the American Heritage Rivers in 1997. The designation provides for technical assistance from federal agencies to state and local governments working in the Susquehanna watershed.
I think that for 4 days, it will not make you grow extra body parts. But, if anyone is pregnant, avoid it. With the potential radioactive element (iodine, cesium, etc) danger from Japan (infered from what they are NOT telling us,) I would suggest a good filter, probably with a charcoal liner.