Help Needed: Stream gauge

Why not the equivalent of a ship's Lubber line. Mark the pipe with black rings at the resolution intervals you require. Either count rings or mark them to know the depth. Fancier systems use wood flats to allow nice lines and numbers.

The design should start with what you want the device to do. Form follows the function. Are you going to personaly monitor the gauge or do you want it to signal you at a predetermined event(when it reaches near flood stage)? The first part you need to determine is where and how you are going to affix the gauge. The perforated pvc is a good idea. Inexpensive, durable, easy to work and replace. An easy way to signal high water level is to place a smaller tube of pvc inside the perforated one. Cap both ends and determine what range you want the signal to appear. Paint the top red or white, whatever colour is easier to see. when the water pushes the inner tube up, the colour will show. there is some other math to do but you get the idea.

This method would require a compressed air source (which could be a compressed air cannister for a limited time run or a better an oil-less air compressor with a tank blow down device). Run a length of plastic tubing to the bottom of the stream and anchor it. Supply compressed air to the tubing through a .005 inch restrictor, available from pneumatic supply companies. Air consumption will be .011 SCFM (standard air cubic feet per minute) If the compressed air is not instrument grade, run it through a filter first. An inline gasoline filter will work. Tee into the line after the restrictor and run the line to a pressure gage. For a maximum 20 ft depth, a 10 psi gage will work. For each foot of water depth, the pressure at the gage will increase about .43 psi. If you want to log the level, use an electronic pressure sensor with a 4-20 ma output and tie it to a portable data logger such as made by Onset Computer Co.