The west side of Brace Mountain drains into the Noster Kill, then into Bash Bish Brook, the Roeliff Jansen Kill, thence the Hudson River and the Atlantic Ocean. The southwest side drains into Webatuck Creek, Ten Mile River, thence the Housatonic River and Long Island Sound. The southeast side drains into Riga Lake and South Pond, then into Wachocostinook Brook, Salmon Creek, and the Housatonic River. The north side drains into Ashley Hill Brook, thence Bash Bish Brook, and the Roeliff Jansen Kill.