Having made about seventeen feet of stream some time ago I needed some bridges. I decided on three. They needed to be small, so simple humpbacked bridges seemed the best route. I chose to go simple. I chose to go cheap. I chose to play a trick. Then I built them, fell ill and didn't post this as originally intended - fortunately I remembered to take the memory card out of my camera so that I didn't accidentally erase the pictures; erasing pictures on the camera card is something I do regularly to speed up the find and download process.
You don't need many materials or tools to make these things, and those materials you will need are useful for many other things - so no pointlessly wasted materials left over. Cheap as chips - £0.50 each, tops.