Capitalization: 'rivers'

In the following sentence, is rivers capitalized or not?

The Delaware and Potomac rivers are beautiful.

This is a style question. Some style guides specifically recommend lowercasing the plural form rivers in exactly the situation that the OP raises. For example, Chicago Manual of Style, Fifteenth Edition (2003) has this:

8.57 Mountains, rivers, and the like. Names of mountains, rivers, oceans, islands, and so forth are capitalized. The generic term (mountain, etc. is also capitalized when used as a part of the name. In the plural, it is capitalized when it is part of a single name (Hawaiian Islands) and when it is used of two or more names both beginning with the generic term (Mounts Washington and Rainier). When the generic term comes second and applies to two or more names, it is usually lowercased (the Illinois and Chicago rivers).

And the Associated Press Stylebook (2002): has this:

river Capitalize as part of a proper name: the Mississippi River. Lowercase in other uses: the river, the Mississippi and Missouri rivers.

Other style guides offer no guidance on this point, and still others may recommend capitalizing the plural word. Your best bet is to find out which guide (if any) your publisher or institution generally follows, and to adjust your style accordingly.