The main problem with rivers of Lead, is that you need a process to return the lead to the highlands again. SO you'd need an atmosphere, clouds, rain/snow of lead.

Transport processes of some metals appear to be active on Venus with a metallic "snow" in the highlands there, but of course Venus has a much more substantial atmosphere.

ON Mercury, there is no plate tectonics, little sign of recent volcanism (but wait for more MESSENGER images before we draw the line on this one), and a very thin atmosphere.

There *might* possibly be lakes of lead that melted on hot days like asphalt on Earth's roads, but very unlikely to be rivers