you can create a entity with med (your river) i would do it like this: create a terrain, save is as an model, delete the vertices you don't need and save it again so you have a goodlooking river ent.
Check this out... you will love it... Do an internet search for "MystyMood" by "Loopix" You will find an awesome demo in both C-Lite and C-Script... Download the demo of your choice and investigate it. You will get all the code and art work and it's free to use... no royalties... I use it in my projects. It has an excellent River and Sea and Fog and Terrain with a very cool system for rendering. You will learn a lot and love the code... You might even want to do a save as of the entire project and swap out what you don't want for what you do want. G'd Luck...