Morgulduin

Morgulduin was a river of Ithilien, which began in the Ephel Dúath and flowed through the Morgul Vale, under the ghastly bridge that led into Minas Morgul, and eventually into the Anduin. Because it originated in the Morgul Vale, it was icy-cold and its waters were polluted and not safe to drink from. According to The Atlas of Middle-earth, the river was less than twenty-five miles in length.
In the year TA 3019, Faramir warned Frodo and Sam not to drink from it, as the water was tainted and would harm whoever imbibed it. The Morgulduin appears as a wide, polluted river adjacent to Minas Morgul.