TheTerrainguy.com has released the Flexible River System terrain set.
From their announcement:
A unique one-piece river system in lengths of from 2 ft. to 6 ft allows you to create a river that stretches completely across your game table from one side to the other without the need to have many different small river segments. When the game is over, simply roll up the river piece and store until time for the next game.
This river is made with a thin dense foam rubber base and covered with rubberized coatings and flocked. This river is flexible yet durable and has enough weight to allow it to lay flat on the table without curling up on the edges.
Choose from six different widths ranging from 1 inch wide to 5 inches wide, and in lengths of from two to six feet and is available with a variety of styles of river bank to match our gamemats.