Clone555, it depends on what type of river you have in mind. There are various types of river beds and tanks made into looking like these river beds. You'd have to decide what river bed you're into in particular, then research on how to make your tank look like the particular river bed. There are many articles on the internet as well as many books on the subject. I'd recommend to you a book called "Aquarium Designs Inspired by Nature" by Peter Hiscock. The book is wonderfully illustrated and covers a really wide range of biotopes to recreate in a home aquarium. I'm sure you'd find something you'll like in this book. In addition, it provides information on how to make your tank look like a particular biotope (or a river bed in your case).