How is a staff gage installed in a reservoir? The typical way is to drive a piece of 2″ galvanized pipe into the ground, deep enough to so it isn’t easy to push over. If cattle will be in the reservoir to get water, then the pipe needs to be really well installed. A gas-powered post pounder can be rented at Rental Guys, Home Depot, or similar places.
Most reservoirs are deeper than six feet, so it’s best to maximize the length of pipe installed. The length of pipe that can be installed by hand is usually about 6 feet. For a 6-foot tall pipe, about 3 feet of pipe needs to be in the ground, so the total pipe length is 9 feet.
Then the staff gage is attached to a 2″ x 8″, using screws or small bolts. Staff gages vary in width from 1″ to 4″; the usual USGS Style C staff gages are 2-1/2″ wide. Once the staff gage is screwed on, the board is U-bolted to the pipe.
That’s it…except for the surveying part. The top of staff gage needs to be at the same level as the spillway crest, so the maximum water surface elevation can be measured. How is the surveying done? An autolevel is close enough for most reservoirs, or two installers can use a very accurate survey level.
If the reservoir is deeper than your staff gage length (6 feet as shown here), and most are, then staged staff gages will be needed. For example, a first gage is installed at the top, going from, say, 6 feet to 12 feet. The second, lower staff gage is installed from 0 feet to 6 feet, and 6 feet is exactly the same elevation on both staff gages. In the photo below, there are 3 staged staff gages to measure 18 feet in elevation: 0 to 6 feet, 6 feet to 12 feet, and 12 feet to 18 feet. The top of the third, lowest staff gage can be seen in the bottom right corner.
More commonly, staff gages like the USGS Style C are purchased in 3.33 foot lengths. This is convenient because staff gages are installed closer together.
What if a pond is full, or mostly full? It is still possible to install a staff gage, but it will be harder. Boats or rafts will be needed, and the pipe with the board 
already attached has to be put in place and held vertical while being driven. If the total depth is greater than 6 feet, then a longer pipe, board, and staff gage will be needed, and the combined weight will be that much greater. Hint: tie a rope and buoy to the pipe so when if it slips and sinks, it can be pulled up again.
What about installing a staff gage along the slope of a dam, to avoid having to wrestle a pipe and board for a deep installation? This can be done by attaching a length of rebar or pipe to the dam face using concrete stakes or similar method. The slope distances measured are converted to vertical depths. However, this won’t stand up well to cattle or elk traffic, and it is more liable to be vandalized if the reservoir has easy access.