High water recorders are easy. They are called cork tubes.
A tall narrow (hopefully transparent or translucent tube) is firmly attached to the place you want to record the stage. A bit of ground up cork is put in the bottom. By looking at water lines of other floods, you can get an idea how long a tube you need. Make sure the tube is a bit taller than you think the water will get.
http://tinyurl.com/3exoyl Look at pages 18 and 19, for detailed instructions how to make this device, called a crest gage. Doesn't take more than about $10-15 of materials
The ones I've seen aren't this elaborate. They consisted of a burnt out 6 or 8 ft. office fluorescent tube, with the top end cut off smoothly with a glaziers tool, and a bunch of little holes carefully drilled in the base. Dump in ground cork. Set tube into flooding area, calibrated for elevation, but with the cork dry. Go away. Wait for rain. Come back. Look at the tube with a flashlight and see how high the water got by the cork ring sticking inside the tube, by measuring the tube.
I know, rube goldberg science. But it works! Needless to say, a glass tube would need to be securely anchored and out of the main flow of the water. This is best accomplished by wrapping the tube in used bubble wrap, and securing the bubble wrap to itself and to the cave with duct tape. As members of the SCA know, you can't hurt much, not yourself or a sword, with it wrapped in bubblewrap and duct tape. You just need to unwrap the bubblewrap before reading the tube. Disclaimer: This invention preceeded Beekman's World by several decades, and I've actually seen one in use on a spring.