Miniature wargamers need rivers. It's a primary need, common to lots of different periods and genres.
However, what about a blood river? In addition to their standard line of river products, Gale Force Nine also makes some specialty river products. One of these is Blood River, a box of red-colored river pieces.
As the box's back shows, the set provides six river pieces – enough to produce almost 6' of bendy red river.
Inside the big box are six items wrapped in white paper, taped shut.
As you can see above, the pieces have a glossy red coat where the 'blood river' flows. The bubbles are sculpted into the piece, and are quite solid.
The pieces are made of a thick flexible material, painted black on the backsides.
Five of the pieces are S-shaped curves, making a river that bends around as it follows the same direction. You can't make the river bend to a new direction, or split, or flow into a pond.
The sixth piece is a straight river piece with a small island. Could be used as a ford.
Here's a close-up with my 28mm zombie fighter for scale, showing how the pieces fit together.
One minor flaw is that, because the red is slightly sticky, debris adheres to it. You can see some bits and grit embedded in the river, fresh from the factory. Some will brush off, but be careful or you'll scratch the river surface.
Of course, it's up to you whether this red river actually flows with blood. Or for sci-games, maybe it's a lava river? For Pulp games, an algal bloom?