The Monongahela River is the river that passes through the The Pitt under The Pitt Bridge. It flows west and empties into the Allegheny River which then forms the Ohio River. Its waters are highly radioactive, up to 600+ rads, making swimming a somewhat impossible task without dying of radiation poisoning.
The shoreline and water are much, much more irradiated than any body of water in the Capital Wasteland (around 350 on the river bank and around 600 in the water). Drinking the water yields another 450+ rads, but no health. In general, the farther east or west you go (in the river) that rads will increase, you may notice up to 2100 rads/sec.
Mona fell into the Monongahela River, causing her skin to be covered in peeling skin and lesions.