The USGS Water Science School
The metal pipe bolted to the side of the bridge in this picture is a crest-stage gage. It is a widely used low-tech piece of equipment that provides valuable information about how high water in streams get during storms.
There are holes drilled in the bottom of the pipe to allow water to enter. Inside the pipe is a wooden rod with markings in feet and inches. The technician puts some ground cork in the pipe and when it rains both the creek water and the water in the pipe rises, thus floating the cork in the pipe. When the water stops rising, and then falls, the cork sticks to the wooden rod at the highest point where the water was.
This device allows the technician to pull the rod out of the pipe and make a quick estimate of the highest gage height the stream reached during a storm. Gage heights are important because it is used to determine stream discharge, which is the amount of water flowing in a stream at any particular moment.
How the stream height (stage) relates to streamflow
How streamflow is measured
Real-time USGS streamflow data