Update

The on-call maintenance guy called back 45 minutes later & helped me find the water valve to the heater, so the torrent is stopped. The landlord called next, and promised plumbers in the morning. He was not entirely pleased with my inability to tell him exactly what was leaking, but hey, it wasn't my idea to jam the heater in behind the stove.

Blessing count: I got my beautiful kitchen rug out of the kitchen before water reached it; the cold water flow is unaffected; there is wine about the place.

Happy Wednesday, everyone.